Two days after a bloody shooting at a busy San Francisco parking garage, the suspected gunman remains on the loose.

It all started at 6:30 p.m. Wednesday at the 5th and Mission Parking Garage near the Westfield Mall and the Metreon. According to San Francisco Police Department spokesperson Sergeant Michael Andraychak, a man was in line in the garage lobby waiting to pay for parking when a man in his 20s shot him numerous times from behind.

"It doesn't appear to be a random shooting. We believe that they may have known each other,"SFPD Officer Carlos Manfredi told ABC7.

“This happened very quickly and sporadically. From what I understand there was no communication between them," he told the Chron.

"I just heard a bunch of shots. I think it was like three, four shots, then everybody outside just started running inside, everybody. It was crazy," one witness told ABC7.

"The shots kept on going on and everybody started screaming and ducking for cover like a bomb had gone off," another witness said. "We saw the victim, by that time, come running out in the street and fell down, obviously had been shot."

The man, who was struck multiple times in the "lower extremities," was transported to San Francisco General Hospital and is in critical but stable condition.

Police, believing that the suspect had fled into the parking garage, closed down the seven-floor, 2,500-space structure for about two hours, to no avail.

Manfredi told KTVU that "We took all the public outside. We used our tactical units to do a systematic search, floor by floor, to check all the vehicles in the parking garage to make sure that the suspect wasn't still outstanding and on the loose."

Police didn't find a trace of the shooter, however, and must now turn to the tape.

"We're reviewing all of the video surveillance video that we captured," Manfredi says, "and we're also doing systematic checks throughout the other buildings, block by block."

Know anything about this shooting? If so, you're urged to call SFPD's Anonymous Tip Line at 415-575-4444 or to text TIP411 with “SFPD” at the start of the message.