A United flight had to return to the Denver International Airport gate Thursday night because of three suspicious passengers on board.

The flight was United Flight 593 to Orange County.

According to reports, Denver Police boarded the plane after it returned to the gate and arrested four people. Everyone was taken off the plane for a security sweep.

According to the Transportation Security Administration, the four suspects were questioned in the terminal and were released. Witnesses described the suspects as middle aged. Two are Caucasian and two are of Middle-Eastern descent.

It is unknown at this time why the people were deemed “suspicious.”

The flight departed after about a 2.5 hour delay.

(Source: 9News)