WASHINGTON, Jan 31 (Reuters) - The investigation into a security breach that prompted the evacuation of San Francisco's airport hit snags on Thursday, and the security screener who triggered it by detecting possible explosive residue on the shoes of a passenger was suspended, authorities said. The FBI completed its review of Wednesday's incident which prompted the evacuation of thousands of people from the United Airlines (UAL) concourse and delayed dozens of flights. Lacking a key witness and a crucial evidence, the law enforcement agency turned the case over to the Federal Aviation Administration. Aviation regulators can pursue civil fines. Argenbright Security, a unit of British security services firm Securicor Plc (SCR), suspended the employee who screened the passenger pending an internal review of the incident. Fearing further attacks like those on Sept. 11, the FAA has ordered screeners to perform random checks of passengers' shoes after authorities alleged that Richard Reid, a passenger on a transatlantic American Airlines (AMR) flight in December, tried to blow up the plane with plastic explosives hidden in his sneakers. Efforts to identify the passenger at the center of the incident at San Francisco International Airport were complicated further after a surveillance camera at the screening checkpoint did not produce a clear picture of him to generate new leads, authorities said. According to law enforcement and aviation authorities, an airport checkpoint screener detected a trace amount of what possibly could have been a chemical residue sometimes found in certain plastic explosives on the shoes of the passenger. The residue also could have come from a common lawn fertilizer, officials said. The alert produced by a hand-held security device could also have been a false reading. The shoes were returned to the passenger who left the checkpoint unchallenged. He has not been seen since. Airport officials said it was possible the passenger, described as a businessman in his 40s, never knew anything was wrong. The screener apparently left the passenger at the checkpoint to alert a superior. Argenbright said the test for detecting explosives was performed correctly. But the company said it was not clear if new procedures or training were needed. Securicor is due to close Argenbright after lawmakers ordered the government to take over screening. Two of the hijacked planes on Sept. 11 took off from airports where Argenbright provided security. POSSIBLE DELAY IN EVACUATION Adding to the controversy, the FAA complained that its evacuation order, issued at about 7:30 a.m. PST/10:30 a.m. EST, was delayed for up to an hour because its security chief in San Francisco was not promptly notified of the incident. An FAA spokeswoman said the agency's security manager, who gave the order, was notified while driving to work and immediately instructed that the bustling terminal be cleared. The airlines are responsible for passenger and baggage screening, and United disputed the assertion that proper security officials were not promptly informed. "We're not going to accept that on face value," said United spokesman Joe Hopkins. "There was a question of whether that was true or not. That is part of our investigation." The FAA said it would investigate to see if any of its regulations were violated. The Transportation Department will take over responsibility for all airport passenger and baggage screening on Feb. 17.