The US justice department has launched a civil rights investigation into Major League Baseball after the league criticized three San Francisco Giants players who wrote Bible verses on their hats during the team’s Pride Night.

Most of MLB’s 30 teams celebrate Pride month with a themed game to acknowledge the LGBTQ community and its baseball fans. During a 12 June game against the Chicago Cubs, pitchers Landen Roupp, JT Brubaker and Ryan Walker wrote Bible verses on their hats, which featured the Giants’ logo in rainbow colors, while pitcher Sam Hentges chose not to wear the themed cap at all.

MLB said in a statement on Monday that writing on hats “violates our rules, and consistent with normal practice, we have warned the players about future violations.”

On Thursday, assistant US attorney general Harmeet Dhillon wrote a letter to MLB commissioner Rob Manfred saying the justice department had referred the league to the US Equal Employment Opportunity Commission to investigate whether the discipline amounts to religious discrimination.

“The Civil Rights Act prohibits MLB and its franchises from unreasonably burdening the rights of players with religious objections to serving as the League’s vehicle for pro-Pride messages,” Dhillon said in the letter. “Federal law is clear: employers must modify their uniform requirements to reasonably accommodate their employees’ exercise of religion.”

Dhillon called MLB’s decision to allow players to wear Black Lives Matter uniform patches in 2020 a “double standard”. Andrea Lucas, the EEOC chair, reposted Dhillon’s letter saying the agency could not confirm the existence of a charge or investigation without a court filing or public resolution, but added: “Rest assured, however, that EEOC is committed to protecting the religious liberty of all workers.”

Roupp said after the Giants’ game last week that the decision to write on his cap was not malicious and that there was “no hate at all”. Hentges said he did not appreciate being told to wear the cap for a cause he did not “morally support”.

After MLB’s warning to the Giants players, vice-president JD Vance weighed in on X, saying: “Trump won we don’t have to do this anymore”, referencing Pride hats. Republican senator Josh Hawley wrote a letter to Manfred voicing his concerns over what he termed a “pattern of discrimination” against Christian players.

The league has said that its warning about writing on caps has nothing to do with the content of the message, and that it has sent the same warning for Mother’s Day messages and names of family members.

The controversy over Pride nights is not new in baseball. In 2022, several members of the Tampa Bay Rays refused to wear the team’s rainbow-themed logos in “faith-based decisions”. This week, it extended beyond the majors, as the independent league York Revolution forfeited a game after some of its players refused to don the Pride Night jerseys.

The Giants released a statement after their Pride Night saying they are “proud to support Pride Night and the LGBTQ community” while also respecting that “individuals may make personal choices about participating in team activations” and apologizing for the “pain and anger [of] many in the LGBTQ+ community”.

San Francisco has a large LGBTQ population and holds an important place in the history of American LGBTQ rights. The director of the city’s Pride efforts told NBC Bay Area that the moment was painful for many Giants fans.

“I hate that it’s dividing us, this time of year I hope that we’re unifying people,” said Suzanne Ford.

“It’s so ludicrous that this story is being spun that Christians are being discriminated against,” she added.