SAN FRANCISCO - Thousands of people were evacuated from San Francisco International Airport on Wednesday after security guards detected possible explosive residue on the shoes of one passenger, but let him vanish into the crowd before he could be questioned.

The alert -- which emptied parts of a major domestic terminal at one of the nation's busiest airports -- left both officials and passengers questioning the efficiency of new security measures designed to bolster confidence in the air travel network.

"How he got away is a question that we all have," said airport spokesman Ron Wilson.

Random shoe tests were ordered by the Federal Aviation Administration in December after passenger Richard Reid, 28, allegedly attempted to set fire to a pair of explosive sneakers on an American Airlines flight traveling from Paris to Miami. Increased air security has also been implemented since the Sept. 11 hijack plane attacks on New York and Washington.

The San Francisco breach occurred shortly before 7 a.m. (10 a.m. EST/1500 GMT), when a businessman going through a random security check tested positive for some kind of explosive on his shoe.

"They screened his shoes by doing a trace wipe on them. The wipe came back as a possible positive hit of residue of some type," airport official Mike McCarron said. "When they went to ask the gentleman some more questions he was not there anymore. He was lost in the crowd."

Officials said the man took his shoes off for the screening but they were returned to him. The test subsequently turned up traces of chemicals present in both certain kinds of plastic explosives and common lawn fertilizer.

In San Francisco, officials ordered the boarding area cleared, shutting some 30 gates in the airport's Terminal 3 and delaying as many as 80 domestic flights of United Airlines, the airport's busiest carrier.

In addition, passengers already aboard a number of jets were told to get off and go through security screening procedures again.

But the alert apparently did not come quickly enough to stop as many as six planes, which had already been loaded, from taking off.

Wilson said "word had not gotten down" to the flight crews and dispatchers that something was amiss in the terminal, but that two flights were subsequently diverted to nearby airports where the passengers were screened again.

With as many as 3,000 passengers facing delays of two to three hours, a major air traveler group voiced concern that the security mess and the missing passenger had exposed dangerous holes in the nation's air safety program.

"This is extremely troublesome," said David Stempler, president of the Air Travelers Association.

"It has got to be a layered system, with lots of layers of security to try to stop a criminal or terrorist from getting on airplanes," he said. "The very last thing we want is to have a terrorist get on an airplane. We now know the results of that."

But despite the massive security sweep, authorities said they had turned up no sign of the passenger, described only as a white male in his 40's.

McCarron said it was unclear if the man was even aware that his shoes had raised a security alarm.

"He may not have even known they wanted to talk to him ... but he was lost in the crowd rather quickly. It's a very busy time here at the airport," McCarron said.

Officials said the man's shoes had set off an "explosive trace detector," a piece of equipment set to scan for traces of eight or nine different types of explosives.

"It doesn't necessarily have to be a military explosive," Wilson said.

The FBI questioned the guard, who was employed by Argenbright Security, the same firm fired last year at Boston's Logan Airport after security lapses there.

Wilson said the guard apparently violated standard procedure by returning the man's brown loafers to him after the test and then leaving to look for a supervisor. The passenger, evidently believing he had passed inspection, put the shoes back on and left, he said.

"We're going to have to figure out why that occurred," Wilson said. Nampa-Reuters