(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.

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