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Chicago and federal authorities are searching for answers to a host of vexing questions about security at O’Hare International Airport in the wake of Thursday’s massive evacuation of air travelers because of a security breach.

Primary among those questions–and the most difficult to answer–is this: Who was this man who, according to a privately hired security guard, burst through a security checkpoint and vanished into a sea of travelers?

Complicating the investigation is the fact that, according to a police report drafted after Thursday’s incident, there was only one eyewitness to the security breach–the security guard who reported it.

“Whoever he is, Mr. Whipple or whoever, he’s at home watching all this on TV, and he’s probably thinking, `I caused all this?’ ” said Monique Bond, a spokeswoman for the Chicago Department of Aviation.

Chicago Aviation Commissioner Mary Rose Loney, while defending the airport’s security procedures, conceded Friday that the man might never be identified. He was described as slim, with an olive complexion, short hair and about 6 feet tall. He wore a long-sleeve green shirt, tan Dockers slacks and was carrying a tan canvas briefcase.

“The Chicago Police Department and the (Federal Aviation Administration) will do their best . . . but the reality is he may never be found,” Loney said.

Authorities are studying security camera videotapes of the area, Loney said, although airport security analysts say the tapes may not yield a clear image.

More than 6,000 people were evacuated from a United Airlines terminal Thursday afternoon when a private security guard reported to police that a man burst through a checkpoint. Authorities were concerned that the man might have carried explosives in the briefcase.

The incident disrupted air travel for more than 27,000 passengers worldwide. It took until late Friday before travel at O’Hare returned to normal, officials said.

Mayor Richard Daley said he has asked Police Supt. Terry Hillard to review security at O’Hare and suggested that some exit doors be locked to thwart those seeking to go through from the wrong direction.

An FAA official said Friday that an initial review of the situation gave the agency “every reason to believe (the evacuation) was a prudent response by all the parties involved.”

The O’Hare incident follows warnings from the U.S. Department of Transportation that security has not been sufficiently improved at major U.S. airports during the 1990s.

A 1993 audit by the department’s inspector general found FAA security procedures at five unnamed major airports easily penetrable. When transportation officials revisited the issue this year, they reached the same conclusion.

“They have not improved that much since 1993,” said Jeff Nelligan, a spokesman for Inspector General Kenneth M. Mead.

In testimony before a congressional committee in March, Mead called FAA oversight of access to secure areas “inadequate.”

Federal officials will issue another report this fall detailing security shortcomings uncovered by transportation officials at U.S. airports, said Nelligan. Much of the report will focus on security personnel hired by the FAA to guard secure areas, such as the baggage-loading areas and control towers.

Chicago police, United Airlines’ security officials, the FAA and the city’s Aviation Department all will be conducting investigations of the incident, jointly in some cases, spokesmen said.

O’Hare will review all its security measures, including the possibility of erecting security screening areas between concourses within a terminal.

“Everything is on the table,” said Richard Kunicki, the city’s deputy aviation commissioner who oversees security at O’Hare. “We’re reviewing everything that occurred, and we will be working on it through the weekend.”

There also has been concern by federal officials–including the FAA–over the training, qualifications and pay levels of private security guards hired by airlines to conduct security screening of travelers and others who pass into the airport’s concourses.

The guard who reported the man breaking through security on Thursday was trained according to federal guidelines, according to his employer, Atlanta-based AHL Services Inc.

“We are very satisfied that his response was completely the best thing that could have been done,” said Celeste Bottorff of Argenbright Security, a subsidiary of AHL.

In 1997, a commission on aviation security headed by Vice President Al Gore called for an upgrade in the training of private airport security workers.

Loney said Friday that she recommended to the Gore commission that such workers undergo additional on-the-job training. “That has not happened,” she said.

Airport security screeners employed by Argenbright receive two days of training before beginning work, which is followed by on-the-job instruction and monthly and annual reviews, said an Argenbright official in another city, who declined to be identified.

Depending on the contract with the client and the work being performed, pay for Argenbright security guards can range from minimum wage to $10 per hour, the official said.

Officials of United Airlines and the airport all said Friday that the O’Hare guard appeared to have followed all FAA security requirements and more: He called for the man to stop; gave pursuit, although he was not required to do so; and promptly notified police and Department of Aviation officials.

“I don’t think the human factor failed,” said Robert Weitzel, a principal in International Total Services in Cleveland, an airport security firm that operates at O’Hare’s international terminal.

Some observers have raised questions about having more than one private agency perform security functions at the airport, and the FAA will issue new rules this fall under which it will certify the private contractors, such as AHL.

Also under review is how many security checkpoints travelers must pass through. O’Hare was among the first airports to have terminals in which passengers are required to go through only one screening area leading to all concourses.

Separate security screening checks for each concourse would avoid having to evacuate the entire terminal in the event of a security breach.

“One of the contributing factors to the size of the problem was the size and extent of the terminal,” said Richard Marchi, senior vice president for technical affairs at Airports Council International, an industry group.

Thursday’s incident was not the first of its kind requiring evacuation of an airport terminal.

Last November, more than 1,000 passengers and others were evacuated from a terminal in Los Angeles International Airport after a rowdy group of people crashed through security screening gates.

In Detroit, also last year, three security breaches at Northwest Airlines’ terminals grounded scores of flights and forced the evacuation of several thousand people.

Friday morning, United canceled 75 more flights–both arrivals and departures–and numerous others were delayed.

Overall, United canceled a total of 131 flights because of the incident, a spokesman for the airline said.