WASHINGTON, Feb 4 (Reuters) - A Delta Airlines (DAL) flight from Denver to Salt Lake City, Utah, was forced to turn back on Monday night after a passenger found a threatening letter on the plane. "It took off from Denver and a passenger found a threatening letter on board and they returned to Denver and took the aircraft to a secure location at the airport," said Federal Aviation Administration spokeswoman Laura Brown. She said Delta Flight 1642 left Denver International Airport with 56 passengers and six crew members at 6:45 p.m. local time (0145 GMT Tuesday) and returned 30 minutes later. Passenger Matt O'Mara said that half an hour into the hourlong flight, the pilot announced the plane was turning back to Denver, without giving a reason. "Later, when people started to get up, the air marshall jumped up and identified himself and yelled very loudly for everyone to stay seated," O'Mara said. "That elevated our anxiety." After the plane landed, he said, the pilot said a note that mentioned a bomb had been found on the plane. FAA and Delta officials would not comment on the contents of the note. Brown said the passengers were taken off the Boeing 737 and sent through a new round of security checks while the plane was searched. "They have checked the plane and nothing out of the ordinary was found," added airport spokesman Charles Cannon. Airport officials said the plane took off again at 10 p.m. for Salt Lake City, site of the 2002 Winter Olympics.