The United States Postal Service warned Pennsylvania that mail ballots may not be delivered on time to be counted because the state’s deadlines are too tight for its “delivery standards,” casting fresh doubt on Pennsylvania’s ability to conduct much of the 2020 election by mail.

The warning came in a July 29 letter from Thomas J. Marshall, general counsel and executive vice president for the Postal Service, to Pennsylvania Secretary of State Kathy Boockvar, whose department oversees elections. That letter was made public for the first time late Thursday in a filing the Pennsylvania Department of State submitted to the state Supreme Court, in which it asked the court to order that mail ballots be counted as long as they are received up to three days after the Nov. 3 election.

If the court agrees, it will increase the likelihood that the results of the presidential race between President Donald Trump and presumptive Democratic nominee Joe Biden won’t be known for days after the election.