A letter sent late Friday afternoon by the Trump administration is casting doubt on the future of the Gateway rail tunnel project.

An official at the nation's federal transportation agency wrote that the plan to pay for the estimated $13 billion dollar rail tunnel under the Hudson River raises serious concerns—including that it relies on a "non-existent" agreement that would have the federal government foot half the bill.

In the letter to New York and New Jersey officials, K. Jane Williams, the deputy administrator of the Federal Transit Administration, criticized the states' reliance on a 50-50 funding agreement with the federal government dating back to the Obama administration.

President Donald Trump is expected to announce his infrastructure plan next month, and it's not clear where the Gateway project fits in.

A spokesman for the Gateway Development Corporation, which is composed of representatives of both states and Amtrak, and oversees the project, says his agency remains confident the project will move forward.

Amtrak, which owns the tunnels and most of the tracks along the corridor, estimates the existing tunnel could fail within 10 to 15 years due to saltwater damage caused by Sandy in 2012.