SAN FRANCISCO -- A laptop computer containing personal information from 33,000 travelers who applied to a program for bypassing airport security lines was probably stolen and returned, not just misplaced, investigators said Monday.

The Transportation Security Administration announced last week that it had suspended new enrollments to the program, known as Clear, after the unencrypted computer disappeared from a locked office at San Francisco International Airport.

The day after TSA's announcement, the laptop reappeared in a cabinet in the same office.

Verified Identity Pass Inc., which runs the Clear program, said at the time that the company did not know whether its computer had been stolen or just overlooked.

Investigators are now treating the disappearance as a theft and plan to interview Verified Identity Pass employees to figure out who took the laptop and why, said San Mateo County Sheriff's Sgt. Wes Matsuura.

It was "highly doubtful" that a random member of the public swiped the computer, then returned it to the locked office, Matsuura said.

The Clear program allows passengers to pay to use special "fast lanes" to avoid long lines at airport security checkpoints. They still must undergo normal security screenings once they reach the checkpoint.

The laptop contained applicants' birth dates and, in some cases, driver's license, passport or green card numbers, but not Social Security numbers or credit card numbers, the company said.

The TSA is trying to determine whether anyone attempted to access the personal information, Matsuura said.