Donald Trump is expected to sign an executive order on Thursday launching a commission to review alleged voter fraud and voter suppression in the US election system, three White House officials said.

Mike Pence and Kansas secretary of state Kris Kobach will lead the commission, which will look at allegations of improper voting and fraudulent voter registration in states and across the nation, one official said. The officials spoke on condition of anonymity because they were not authorized to discuss details ahead of the formal announcement.

Trump has alleged, without evidence, that 3-5 million people voted illegally in the 2016 election. He has vowed since the start of his administration to investigate voter fraud, a process that has been delayed for months.

The commission will include Republicans and Democrats and be composed of current and former state election officials and other experts, the White House official said.

The panel will aim to ensure confidence in the integrity of federal elections while looking at vulnerabilities in the system and the possibility of improper voting and fraudulent voter registration and voting, the official said.

Potential panel members include former Ohio secretary of state Ken Blackwell and Indiana secretary of state Connie Lawson, the official said.

Other names under consideration include longtime New Hampshire secretary of state Bill Gardner and Maine secretary of state Matthew Dunlap, both Democrats, and Christy McCormick of the US Election Assistance Commission.

During his campaign, Trump repeatedly alleged that the election system was “rigged” and after his election argued that such massive, widespread fraud kept him from the popular vote. Trump won the presidency with an electoral college victory despite losing to Clinton by nearly 3 million votes.

Voting experts and many lawmakers have said they haven’t seen anything to suggest that millions of people voted illegally, including House oversight committee chairman Jason Chaffetz. The Utah Republican said his committee will not be investigating voter fraud.

In a lunch meeting with senators in February, Trump said that he and former Republican senator Kelly Ayotte would have won in New Hampshire if not for voters bussed in from out of state. New Hampshire officials have said there was no evidence of major voter fraud in the state.

Trump had previously identified Pence as the person to oversee the long-awaited commission. Kobach advised Trump’s transition team and has been a leading GOP proponent of tighter voting regulations.

The secretary of state championed Kansas’s proof-of-citizenship requirement as an anti-fraud measure that keeps non-citizens from voting, including migrants living in the US illegally. Critics contend it suppresses voter turnout, particularly among young and minority voters, and that there have been few cases of fraud.