Former President Barack Obama warned on Tuesday that some political leaders and their allies are using the assassination of conservative activist Charlie Kirk to widen divides. 

Speaking at the Jefferson Educational Society’s 17th annual global summit in Erie, Pa., Obama said recent political attacks and threats of drastic action in response to Kirk’s killing have worsened tensions in the country.

“When I hear not just our current president, but his aides, who have a history of calling political opponents ‘vermin,’ enemies who need to be ‘targeted,’ that speaks to a broader problem that we have right now, and something that we’re going to have to grapple with — all of us,” he said.

Kirk was shot and killed last week at a university event in Utah. Prosecutors on Tuesday formally charged Tyler Robinson, 22, with Kirk’s murder and said they would seek the death penalty. Charging documents said Robinson sent text messages confessing to the shooting and wrote, “I had enough of his hatred.”

Obama called Kirk’s death “horrific and a tragedy,” but said Americans must remain able to debate Kirk’s ideas.

“Whether we’re Democrats, Republicans, independents, we have to recognize that on both sides, undoubtedly, there are people who are extremists and who say things that are contrary to what I believe are America’s core values,” he said.

The former president said moments of national crisis require leaders to act as unifiers. 

“I think George W. Bush believed that,” he continued. “I believe that people who I ran against — I know John McCain believed it. I know Mitt Romney believed it. What I’m describing is not a Democratic value or Republican value. It is an American value. And I think at moments like this, when tensions are high, then part of the job of the president is to pull people together.”

He said the country is experiencing a “political crisis of the sort that we haven’t seen before.”