Singer John Denver was flying illegally when he was killed in the crash of his experimental aircraft into Monterey Bay Sunday, federal officials said Tuesday.

The Federal Aviation Administration had asked Denver to stop flying almost a year ago after learning he had been involved in alcohol-related automobile accidents. The agency had suspended the medical certificate all pilots must have to fly, aviation sources said Tuesday.

Denver returned unopened or did not reply to several certified letters asking him voluntarily to surrender his FAA medical certificate on grounds it was no longer valid. He had been flying illegally since at least March when he finally accepted a certified letter, officials said.

The FAA said it had not yet initiated legal action to require Denver to give up the certificate, issued under his original name, Henry John Deutschendorf. Sources said the agency is looking into why seven months had passed with no legal action. In any case, his pilot’s license was not valid without a valid medical certificate.

Denver was arrested twice in Colorado on drunken driving charges, in 1993 and 1994, and was scheduled for trial on one of them in January. FAA spokesmen refused to say whether the alcohol arrests were the reason for the letters to Denver, citing privacy reasons.

However, aviation sources said Denver apparently acknowledged the arrests when he last applied for a medical certificate. A line on the medical certificate form specifically asks for this information. The FAA then investigates further. If a pilot is involved in at least two drunken-driving incidents over a three-year period, the FAA can request return of the medical certificate and the certificate-holder must reapply with proof he no longer suffers from an alcohol problem.

The National Transportation Board, which is investigating the crash, said Tuesday that Denver was practicing takeoffs and landings Sunday at the Monterey, Calif., airport in a Long EZ experimental aircraft that he had purchased the day before.

George Peterson, the NTSB investigator in charge, said Denver had performed four “touch-and-go” procedures in which a pilot lands and then immediately takes off without stopping on the runway. He then requested air traffic clearance to Monterey Bay.

Controllers informed him that his transponder was not working. A transponder is a radio beacon aboard planes that enhances a plane’s radar image and can report other information automatically to controllers such as altitude. Denver then radioed back, “How about now?” and was told the transponder was working.

Denver did not reply to the controller’s acknowledgement, and the transponder image then disappeared from the radar screen. The plane crashed about 5:28 p.m.

Peterson said the plane’s engine was recovered Tuesday for examination. The rest of the wreckage was to be taken to a hangar.