Homeland Security Secretary John Kelly says President Trump must possess credible evidence that former President Barack Obama Barack Hussein ObamaGOP senator blocks Schumer resolution aimed at Biden probe as tensions run high D-Day for Trump: September 29 Obama says making a voting plan is part of 'how to quarantine successfully' MORE ordered the wiretapping of Trump Tower in New York City last year.

Trump claimed on Twitter last Saturday that Obama had his “wires tapped” in Trump Tower before Election Day last year, making the accusation without offering any evidence.

“If the president of the United States said that, he’s got his reasons to say it,” Kelly said Monday on CNN’s “The Situation Room." "[Trump’s] got some convincing evidence that that took place.”

Kelly admitted he had learned about Trump’s claims from watching CNN, adding he would not guess why the Obama administration might have wiretapped a 2016 presidential candidate.

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“I don’t know anything about it other when I was sitting off-studio here watching CNN,” he told host Wolf Blitzer.

“I don’t pretend to even guess what the motivation may have been for the previous administration to do something like that,” the retired Marine Corps general added.

“Terrible! Just found out that Obama had my ‘wires tapped’ in Trump Tower just before the victory,” Trump tweeted. "Nothing found. This is McCarthyism!”

“How low has President Obama gone to tap [sic] my phones during the very sacred election process,” Trump added in a follow-up tweet. "This is Nixon/Watergate. Bad (or sick) guy!”

Kevin Lewis, a spokesman for Obama, said last Saturday that Trump’s claims about the former president are “simply false.”

“A cardinal rule of the Obama Administration was that no White House official ever interfered with any independent investigation led by the Department of Justice,” he said in a statement.

"As part of that practice, neither President Obama nor any White House official ever ordered surveillance on any U.S. citizen.”

White House spokesman Sarah Huckabee Sanders said Sunday an investigation of Trump’s wiretapping claims should be included in any probe of ties between the president’s 2016 bid and Russia.

FBI director James Comey asked the Department of Justice to disavow Trump's claims that Obama ordered a wiretap, according to a New York Times report.

--This report was updated on March 7 at 7:11 a.m.