With the Electoral College set on Dec. 19 to cement the results of Donald Trump’s presidential win, UC Berkeley statistician Philip Stark is calling for an audit to double-check that hackers did not manipulate the results.

In an op-ed in USA Today, Stark and MIT cryptographer Ron Rivest, both advisors on the U.S. Election Assistance Commission, argue that there are good reasons to conduct a “risk-limiting” audit of the presidential election.

Among them is the conclusion of the Department of Homeland Security and the National Security Agency that Russian hackers attacked the Democratic National Committee and U.S. voter registration databases.

For example, they say, the election results could have been tipped by manipulating the vote count in a small number of jurisdictions in battleground states, such as areas with limited resources to defend against cyberattacks.

Republican Sen. Lindsey Graham of South Carolina has called on Congress to investigate Russian cyberattacks on the election. Meanwhile, a Washington Post–ABC News poll found that 18 percent of voters believe Trump was not the legitimate winner of the election. Hillary Clinton’s lead in the popular vote has grown to 2 million.

A Change.org petition calling for election officials to double-check the results via a risk-limiting audit is circulating online. More than 250,000 people have signed it so far.