The Houston Rockets probably knew that a Thursday night tweet from the team might invite some controversy. Above a photo of superstar guard James Harden wearing a face covering, they wrote, “Mask Up” — a still divisive demand in a nation where masks have been heavily politicized.

But it wasn’t the message that quickly stirred up a frenzy of critics — it was the image on Harden’s mask.

Harden’s face covering featured the “thin blue line,” a pro-police symbol that critics have long claimed also stands for white supremacy and opposition to the Black Lives Matter movement; and a Punisher skull, which has been associated with far-right groups.

Neither the Rockets nor the NBA, which also shared an image of Harden’s mask to their Twitter account’s more than 30 million followers, responded late Thursday to questions about where Harden got the mask or whether it was meant as a political statement.

Earlier on Thursday, Harden spoke favorably to reporters about the protests for racial equality in Houston.

“The way the city just like rallied, it was amazing. I think the world saw it,” he said. “… The march and everything that we’re standing for is very powerful.”

Among those who slammed Harden for wearing the mask were musician Trey Songz, who called Harden’s choice of a mask “certified clown s---,” tweeting that “I’ll say it for everybody who scared to.”

But rapper Young Thug, a friend of Harden’s, stuck up for the former MVP by suggesting he didn’t know what the thin blue line imagery meant, noting in a tweet that Harden doesn’t “have internet so he obviously don’t know what’s right or wrong if he posted something that’s against US.”

Questions about Harden’s mask come as the league tries to navigate a complex reopening plan inside a bubble at Disney World amid a pandemic and mass protests against police brutality in the wake of George Floyd’s death in Minneapolis.

The vast majority of NBA players have strongly supported the Black Lives Matter movement. The NBA agreed to paint “Black Lives Matter” on the courts at the ESPN Wide World of Sports Complex near Orlando, where the league will play, and will allow players to wear social justice messages in place of their names on their jerseys.

Unlike his fellow NBA superstar LeBron James, Harden has never been particularly vocal about politics. Asked earlier Thursday whether he would choose a message to wear on his jersey, Harden demurred.

“I’m working on it right now,” he told ESPN’s Marc J. Spears. “I’m in the process of figuring out if I’m going to put a message on my back and other ways to deliver my (social justice) message.”

Harden’s mask in the photo shared Thursday looks similar to a number of thin blue line designs that are widely sold online.

The symbol has been widely adopted by officers and pro-police organizations, but critics say that it also represents more extreme views.

Thin blue line flags flew alongside Confederate flags at the deadly Unite the Right rally in Charlottesville in 2017, and in Maryland last year Democratic leaders battled with Republican Gov. Larry Hogan over whether it was appropriate to hang thin blue line flags in government buildings. An Oregon county last year paid $100,000 to a black employee who said she was harassed after asking that the flag not be displayed at her workplace.

The skull symbol on Harden’s mask, meanwhile, is adapted from the logo of the Punisher, a violent vigilante Marvel Comics character. The character’s creator has asked police to stop using the logo, noting that the Punisher “represents a failure of the Justice system.”

Harden and the Rockets play their first game in Florida on July 31.