Last January, when federal immigration agents started an immigration crackdown in Portland, Maine, pediatric occupational therapist Xenia Pantos was driving using their spouse's car to work when they saw masked federal agents and vehicles with tinted windows parked in the road. Worried about immigrant community members, Pantos stopped for a few minutes to observe.

Pantos told NPR they stayed at least 10 feet away from the agents and did not interact with them, but noticed an agent taking photos of another observer's license plate.

Hours later, Pantos' spouse, Carly Williams, a nonprofit consultant, said she received a call from a blocked number. A deep male voice on the other end of the line asked for her by name and identified himself as calling from the Department of Homeland Security.

Williams said the caller asked if anyone else drives her vehicle. When Williams mentioned her spouse sometimes did, the caller asked Williams if she knew her spouse had stopped at an incident that morning.

"What he basically said was, 'You should let her know to not do that anymore because people who are doing that type of thing are getting added to a domestic terrorist watch list,'" Williams recalled in an interview with NPR. (While the caller referred to Pantos as "she" and "her," Pantos uses they/them pronouns).

"That was a pretty terrifying phone call to receive, as you can imagine," Williams said.

DHS declined to comment on the couple's account when asked by NPR.

For months, Department of Homeland Security officials have repeatedly denied having a database tracking U.S. citizen protesters or a database of "domestic terrorists", even as anecdotes like what happened to Pantos and Williams suggest federal agents are collecting observers' information in some capacity.

In a previously unpublicized letter sent to members of Congress in April, recently departed acting ICE director Todd Lyons acknowledged the agency gives itself wide latitude to collect information on individuals suspected of potential violations of law, including interference with ICE operations or officer safety matters, and maintains records on people who were never arrested.

In the letter, Lyons denied that ICE maintains a database of protesters or that DHS maintains a "separate, standalone database" of individuals who were encountered but not arrested or detained. But he said at protests that involved alleged criminal conduct, ICE has collected "information to identify individuals reasonably believed to be involved in, or directly supporting, potential violations of federal law and to address officer safety and facility security concerns." The letter said ICE collects "essential biographic and biometric information and situational details."

Lyons wrote: "If individuals who interact with ICE officers are not arrested or detained, any information collected during those encounters is maintained consistent with applicable law and DHS and ICE policies and is treated as an official government record."

NPR is the first news organization to review the letter, which is dated April 21.

It was sent in response to Rep. Maxwell Frost (D-Fla.) and 11 other Democratic members of Congress who wrote to DHS in February asking questions about what data the department collects on protesters.

Civil liberties experts told NPR Lyons' letter appears to be the clearest official acknowledgement yet by federal immigration officials that they may be routinely collecting and preserving information on protesters and observers who are not arrested.

"This letter is evidence of the fact that ICE is knowingly collecting and maintaining official government records on any protestor or lawful observer that its agents claim is potentially interfering with them or threatening agent safety," said JoAnna Suriani, a lawyer at the nonprofit legal and advocacy organization, Protect Democracy.

Suriani is representing Pantos, Williams and other observers in Maine in a federal lawsuit that alleges their First Amendment rights were violated by federal agents who tried to intimidate them by recording their faces and license plates and threatening to add them to a domestic terrorism database.

"Anyone who has seen the videos of our clients' interactions with ICE agents can see they aren't impeding anything and pose no threat to anyone, so why was their information collected?" Suriani said.

Protesters photographed, filmed and threatened with charges

Since the Trump administration's immigration crackdown began last year, peaceful protesters and observers recording federal immigration operations on their cell phones have been threatened with criminal charges for impeding or interfering with law enforcement operations. However, many cases where charges were brought against activists have been dismissed or resulted in acquittals. DHS officials have also previously asserted that recording federal agents and posting the videos amounts to "doxxing" and is a threat to their safety.

Observers in several states, including Minnesota and Tennessee, complained that agents photographed their faces and license plates and later determined their identities and where they lived. Federal agents have access to a suite of surveillance tools, including facial recognition technology, and can access vehicle registration records using a car's license plate.

A number of observers have also said their Global Entry status was revoked after interacting with federal immigration officials. The program is run by U.S. Customs and Border Protection, another DHS agency, and allows expedited processing for pre-approved, low-risk travelers.

In January, a DHS official sent a memo to some federal immigration agents temporarily assigned to Minneapolis instructing them to collect personal information about protesters and agitators, including license plates, identifications and images, according to correspondence reviewed by CNN. NPR has not independently confirmed the reporting.

Frost told NPR he has been concerned about law enforcement tracking protesters since he was part of the Black Lives Matter movement and learned police were collecting information on him and other protesters.

He said while it may be typical for law enforcement to conduct investigations and determine if someone broke the law and then move on, it is concerning if information on people who are exercising their rights is kept by a large federal department.

"That's the concern, is that we have an agency that's been tasked with immigration enforcement having a database … relating to Americans exercising the First Amendment, which is wrong," Frost told NPR.

ICE letter provides nuance after blanket denial

At a February congressional hearing, Lyons denied his agency was surveilling U.S. citizens and said: "There is no database for protesters."

DHS has repeatedly provided a statement to the media that says, "There is NO database of 'domestic terrorists' run by DHS. We do of course monitor and investigate and refer all threats, assaults and obstruction of our officers to the appropriate law enforcement. Obstructing and assaulting law enforcement is a felony and a federal crime. Our law enforcement methods follow the U.S. constitution."

A department spokesperson provided that statement in response to NPR's inquiry asking if the Lyons' letter still reflected current policy, and again in response to a request for comment about Pantos and Williams' account.

At a congressional hearing last week, Secretary of Homeland Security Markwayne Mullin said his department had used facial recognition technology on people gathered outside of Delaney Hall, an immigration detention center in New Jersey that has been the site of recent protests that have led to intense clashes between some individuals and federal agents. Dozens of people have been arrested in connection with the demonstrations, including some who are accused of assaulting federal officers.

"I have zero tolerance," Mullin said in the hearing. "If you verbally assault our officers, you go after our vehicles, you assault our property, you assault one of our officers, we will find you, we will arrest you."

Lyons' April letter began by saying, "U.S. Immigration and Customs Enforcement (ICE) does not maintain any kind of database of U.S. citizens protesting ICE activities." It also asserted that "DHS policies and practices are designed to respect lawful protests and constitutionally protected activities."

The letter continued, "Where individuals decide to go beyond protected speech and commit crimes against federal personnel and property or threaten, or forcibly impede, assault, or interfere with lawful operations, ICE remains steadfast in exercising its authority to investigate and prosecute violators."

While the letter suggested personal information is only collected if there is potential unlawful activity, Scarlet Kim, a senior staff attorney at the American Civil Liberties Union, said the Trump administration has set a precedent of characterizing lawful First Amendment activities as possible crimes.

"We know that very high level officials within DHS and Lyons himself have explicitly equated First Amendment-protected activities like video recording, gathering information about federal agents, and sharing that information publicly as essentially potential criminal acts that threaten officer safety," said Kim, who is representing observers in Memphis and Minneapolis in federal lawsuits against agencies involved in immigration enforcement.

"So their own definition of what potentially violates the law and could trigger surveillance against an individual includes activities that are squarely protected by the First Amendment," Kim said.

While Lyons writes, "DHS is not creating or maintaining a separate, standalone database for individuals encountered that haven't been arrested or detained," Kim said the letter "strongly suggests" that even if DHS does not have a standalone database of U.S. citizens engaged in First Amendment-protected activities, federal agents are likely collecting and maintaining that information in existing data systems.

"He did not deny that, essentially, that information would not be placed in other existing databases," Kim said.

The letter from Frost and his fellow Democrats was addressed to the Secretary of Homeland Security and asked about policies at DHS, but the response came just from ICE, which is just one agency within the department, raising questions about what may be happening in other parts of the department.

The Democrats' letter questioned whether DHS maintains or accesses information from lists or programs called "Bluekey, Grapevine, Hummingbird, Reaper, Sandcastle, Sienna, Slipstream, and Sparta" among others. A January article by independent journalist Ken Klippenstein reported DHS and FBI have secret watchlists with those code names to track anti-ICE and pro-Palestinian protesters, as well as "Antifa." Klippenstein's report cited conversations with two senior national security officials. NPR has not independently confirmed the reporting.

The letter from Lyons said in response: "ICE does not maintain, add, or access information from the programs mentioned in your letter."

Frost told NPR he plans to continue pressing the department as he has many more questions about how the information ICE is collecting is used and how it is shared with other parts of DHS.

Last month, the organization FIRE, which advocates for freedom of expression, announced that it is suing DHS and ICE for access to records on whether it is maintaining a database of protesters.

Maine couple left with unanswered questions

Pantos told NPR they had no idea their information might be collected by federal agents when they made the decision to pull over and peacefully observe that morning in January, and that what they had done was protected by the First Amendment.

But after the unexpected phone call threatening that Pantos could be added to a domestic terrorist database, Pantos said they felt too scared to observe ICE activity again. They worried about their family's safety.

"We are a queer couple, which brings additional risks," Pantos said. "There has been an ICE surge in Portland and I've felt really overwhelmed and powerless."

In March, two months after the incident, the couple drove to Quebec City in Pantos' car to celebrate their anniversary. When they tried to re-enter the U.S., a Customs and Border Protection officer pulled them aside for additional questioning and took their phones and keys for about an hour, they said.

To their surprise, one of the officer's first questions was to ask Williams if she had her car registration with her, despite the fact that they were traveling in Pantos' car. After Williams said she didn't have it with her, the officer asked her to describe her car and to recite her license plate number if she remembered it, according to the couple's account.

"He was clearly looking at a computer screen," Williams said, adding that the officer "seemed to be verifying what I was saying."

The couple told NPR that was the moment they realized their data must have been retained in some kind of federal system after Pantos stopped to observe federal agents in January.

"I have to think, because he asked about Carly's vehicle when we were in my vehicle, that there is some sort of an alert when you run our passports that brings attention to us in a way that it didn't used to before all of this happened," Pantos told NPR.

"I feel really concerned about what has happened with my data and the data of so many other people," Pantos said.