At least three people watched a livestream as gunmen filmed themselves killing three people in Monday's attack at the Islamic Center of San Diego before their own deaths, according to a CBS News analysis of footage from the attack. One viewer urged another to alert law enforcement, but it is not clear if anyone did.

A recording of the attack later circulated online, including on so-called "gore websites" that have previously served as gathering places for admirers of mass shooters.

According to screen recordings reviewed by CBS News, the shooters were on a Signal video call with a user identified only as "Noelle" before and during the attack. The recordings show Noelle had been on the call with the gunmen for roughly 20 minutes before they began shooting.

During that time, Noelle appears to have used a second phone to join a Discord video call with another user, identified as "Otto," positioning the camera so Otto could watch the Signal call in real time. Otto then began recording on his own device, according to footage reviewed by CBS News.

As the attack unfolded, Otto alerted another person on Discord to the live video. "DUDE LOOK," Otto wrote. The person asked where the gunmen were. "IN SOME MOSUQE [sic]," Otto replied. 

"Tell her call the cops," the person wrote, in an apparent reference to Noelle. "Bro, tell her call the cops," they wrote again a minute later. "It doesn't even matter if they aren't in LA just tell her to call them and they can hopefully find out where he is." 

By the time Otto responded five minutes later, the attack was over. One of the teen gunmen, Cain Clark, 17, had shot and killed the other, 18-year-old Caleb Vazquez, before turning the gun on himself. That sequence was also captured in the video.

CBS News could not independently verify the location of the users who watched the video, though details visible in the recordings suggest some of them may have been located far from California, where the attack occurred.

In Otto's screen recording, VodafoneAL, an Albanian mobile carrier, appeared on the device, and the phone's time zone matched local time in Albania, suggesting Otto may have watched from overseas. A time zone visible on Noelle's device was set to three hours ahead of San Diego, suggesting the user may have been in the Eastern time zone. 

Noelle appeared to be in some kind of online relationship with one of the shooters, according to Barrett Gay, a digital research analyst at the Institute for Strategic Dialogue, who reviewed the online activity of the shooters, as well as Noelle's and others who appeared connected to them. Both shooters had been active in fringe communities on mainstream social media platforms, said Matthew Ivanovich, a senior research manager at the organization.

Authorities said they believe the two teens who carried out the attack met online and discovered they were both residents of the San Diego area before meeting in person, CBS News previously reported. 

The pair shared a nihilistic mix of antisemitic, anti-Muslim, misogynistic, anti-Hispanic, anti-gay and anti-trans hate rhetoric in what the FBI called a "manifesto." The document opens with an antisemitic rant repeating the phrase "IT'S THE JEWS." They appear to refer to themselves as the "sons" of the perpetrator of the 2019 mass shooting at a mosque in Christchurch, New Zealand, that killed 51 people, who also livestreamed his attack.

Discord said on Thursday it had preserved information related to the attack and disclosed it to law enforcement, and that it would continue to work in close coordination with authorities.

"We share in the San Diego community's grief and extend our condolences to the families of the victims and to everyone impacted by this tragic violence," a spokesperson for Discord said. "Discord strongly condemns violence and we have strict policies against anyone who supports, promotes, or engages in violent acts. We found no evidence that the livestream of this event originated on Discord."

Signal declined to comment.

The case has drawn attention from researchers who track how online platforms moderate violent content.

"The attack in San Diego is another deadly reminder that any threat assessment in this country has to account for the increasing violence that we're seeing and the online platforms that really enable this to exist," said Oren Segal, senior vice president of counter-extremism and intelligence at the Anti-Defamation League.

A growing number of attackers now prepare their streams and manifestos alongside their weapons, Segal said, calling it part of the subculture of nihilistic violent extremism.

"Certainly in America you can be as hateful as you want to be. We have that protection for freedom of speech here," Segal said. "But when we see incident after incident of violence against religious institutions and at schools that have a through line, which is a set of websites that normalize this in which the attackers, including these attackers are signaling back to these online communities, it makes you wonder whether we need to take a much harder stance legally or otherwise against them."