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(CNN) —  

President Donald Trump will not sign an executive order initiating an investigation into voter fraud Thursday, White House press secretary Sean Spicer said, hours after telling press on Air Force One that the signing would happen once Trump returned to Washington from Philadelphia.

Trump will instead sign the executive order on voter fraud either on Friday or Saturday instead, Spicer said, adding Trump will also sign actions on immigration and national security.

“It will be a follow-up to the announcement yesterday (inaudible) to better understand voter fraud,” Spicer told reporters earlier in the day. When asked what the executive order would specifically include, Spicer said Trump is continuing to work with his senior team on the final details. But a senior White House official told CNN on Wednesday that the probe would likely be carried out through Trump’s Justice Department.

Trump tweeted Wednesday morning that he would launch an investigation into instances of voter fraud, two days after he told congressional leaders of both parties that he believed as many as 5 million illegal votes were cast in the 2016 election.

“I will be asking for a major investigation into VOTER FRAUD, including those registered to vote in two states, those who are illegal and … even, those registered to vote who are dead (and many for a long time). Depending on results, we will strengthen up voting procedures!” Trump wrote.

Neither Trump nor the White House have offered any evidence to back up those claims, which experts say are completely unsubstantiated.

Trump has faced widespread criticism for his remarks, including from some congressional leaders in his own party. Democrats have alleged that Republican efforts in the name of fighting voter fraud has the effect of preventing or delaying legal voters who traditionally back Democratic candidates.

Jason Chafettz, the Republican head of the House Oversight Committee, told reporters on Thursday he doesn’t “see any evidence” of voter fraud.

“But the President has 100,000 people at the Department of Justice, and if he wants to have an investigation, have at it,” Chaffetz added. “I just don’t see any evidence of it.”

The White House’s announcement that Trump would sign an executive order on the matter came as Trump was flying to Philadelphia to join a Republican congressional retreat where the White House and Capitol Hill Republicans will look to align their agendas and formulate plans for major legislative goals in the coming weeks and months.

But those discussions now fall against the backdrop of the President’s unsubstantiated claims of widespread voter fraud in the election that made him president and the investigation he has said he plans to launch into the matter.

CNN’s Dan Merica contributed to this report.