Sen. Tammy Duckworth sent a letter Thursday urging the head of the Federal Aviation Administration to resist any pressure from President Donald Trump to prioritize construction of his planned triumphal arch over aviation safety.

The letter from the Illinois senator, the top Democrat on the Senate’s aviation subcommittee, adds to questions and concerns over Trump’s proposed 250-foot (76-meter) arch for the nation’s capital. Pushed by Trump to commemorate the country’s 250th anniversary, it would be more than twice as tall as the Lincoln Memorial.

Duckworth wrote that the FAA’s initial review of the arch appears to have been expedited and raised questions about whether the president or his White House aides are “already improperly pressuring FAA to prioritize rubberstamping Trump’s vanity arch over public safety.”

Officials are looking to complete the towering edifice within three years, possibly requiring 20 hours of work per day and cranes up to 320 feet (106 meters) tall, according to a National Park Service preliminary report, which Duckworth cited in her letter to FAA Administrator Bryan Bedford.

The agency said it would respond directly to Duckworth.

The arch’s close proximity to the complex airspace of Ronald Reagan Washington National Airport, where a U.S. Army helicopter collided with a commercial jet last year, killing 67 people, was a key concern for Duckworth.