A spokesman for Barack Obama on Saturday rejected claims from U.S. President Donald Trump that the former president had wiretapped him in October during the late stages of the presidential election campaign.

“Neither President Obama nor any White House official ever ordered surveillance on any U.S. citizen. Any suggestion otherwise is simply false,” Obama spokesman Kevin Lewis said in a statement.

Trump had suggested Obama had improperly tapped his phones, without citing evidence, in a series of tweets on Saturday morning.

“How low has President Obama gone to ‘tapp'(sic) my phones during the very sacred election process. This is Nixon/Watergate.

“Bad (or sick) guy!,” Trump said in a series of tweets on his Twitter account early on Saturday.

“I’d bet a good lawyer could make a great case out of the fact that President Obama was tapping my phones in October, just prior to Election!”

Lewis also said that “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.”

The statement raised the possibility that a wiretap of the Trump campaign could have been ordered by Justice Department officials.

The White House did not respond to a request to elaborate on Trump’s accusations.

A Trump spokeswoman said the Republican president is “having meetings, making phone calls and hitting balls” at his golf course in West Palm Beach.

Earlier, former Obama adviser Ben Rhodes strongly denied Trump’s allegations.

No president can order a wiretap. Those restrictions were put in place to protect citizens from people like you,” Rhodes wrote on Twitter.

In one of the Tweets, Trump said the alleged wiretapping took place in his Trump Tower office and apartment building in New York, but there was “nothing found.”

Trump’s administration has come under pressure from Federal Bureau of Investigation and congressional investigations into contacts between some members of his campaign team and Russian officials during his campaign.

Sen. Lindsey Graham, a South Carolina Republican, said he had no knowledge about any wiretapping but is “very worried that our president is suggesting that the former president has done something illegally.

“I would (also) be very worried if in fact the Obama administration was able to obtain a warrant lawfully about Trump campaign activity.”

Graham said it was his job “to get to the bottom of this. I promise I will.”

Several other Republicans again urged an investigation into a series of intelligence-related leaks.

House Democratic Leader Nancy Pelosi ridiculed Trump’s assertions. `

“The Deflector-in-Chief is at it again. An investigation by an independent commission is the only answer,” she wrote on Twitter on Saturday.

Obama imposed sanctions on Russia and ordered Russian diplomats to leave the United States in December over the country’s involvement in hacking political parties in the Nov. 8 U.S. presidential election.

On Saturday, Representative Eric Swalwell, a Democrat who is a member of the House Intelligence Committee, told Fox News that Trump “is not credible when it comes to talking about Russia.”

Swalwell downplayed Trump’s allegation. “I think this is just the president up early doing his routine tweeting, he said.

“Presidents don’t wiretap anyone. These are pursued by the Department of Justice in accordance with the FBI and signed off by a judge.”

Under U.S. law, a federal court would have to have found probable cause that the target of the surveillance is an “agent of a foreign power” in order to approve a warrant authorizing electronic surveillance of Trump Tower.

Several conservative news outlets and commentators have made similar allegations about Trump being wiretapped during the campaign in recent days, without offering any evidence.

Trump’s national security adviser, Michael Flynn, resigned in February after revelations that he had discussed U.S. sanctions on Russia with the Russian ambassador to the United States before Trump took office.

Flynn had promised Vice President Mike Pence he had not discussed U.S. sanctions with the Russians, but transcripts of intercepted communications, described by U.S. officials, showed that the subject had come up in conversations between him and the Russian ambassador.

Trump has often used his Twitter account to attack rivals and for years led a campaign alleging that Obama was not born in the United States. He later retracted the allegation.