Senate Armed Services Chairman John McCain said Monday that President Donald Trump needs to show evidence he was wiretapped at the behest of former President Barack Obama.

“The president said it, and based on something. What was the basis of his conclusion that his predecessor had broken the law by wiretapping Trump Tower?” McCain said. “The American people need to know.”

When the Arizona Republican was told that White House officials said they would not take further questions about Trump’s Saturday morning tweet, McCain said Trump had an obligation given the gravity of the accusations.

“They should answer questions. The American people have a right to know. They can say whatever they want to say. I’m saying the American people have a right to know on what basis the president of the United States said that his predecessor had broken the law by wiretapping Trump Tower,” McCain said.

He also said that it may not be appropriate for Congress to investigate the charge directed at Obama unless Trump can provide evidence.

“If there’s no basis for it, then there’s no reason for an investigation,” McCain said.

Even if investigations by the House and Senate Intelligence panels go forward, Trump’s White House is making no guarantees about accepting the findings.

“It’s his obligation to tell the American people why he made the allegation that he did, on what basis, and that information should be known to the American people — not just Congress, not just a select committee, but to the American people,” McCain said. “Because he has charged his predecessor, according to the former director of national intelligence, with violating the law.”