Incident Probed at Long Beach Airport : Security: Two intruders enter a restricted area. One boards a just-arrived plane. The FAA is looking into possible safety lapses.

The Federal Aviation Administration said Friday that it will investigate possible security lapses at Long Beach Airport after two men acting suspiciously got onto the Tarmac and one made it aboard an Alaska Airlines jet as passengers were leaving.

The McDonnell Douglas MD-80 aircraft was grounded temporarily Thursday evening and searched by police and by bomb-sniffing dogs, but nothing was found, authorities said. Both men fled before airport security officers arrived.

An Alaska Airlines spokesman said 55 passengers who were to have boarded the plane for a flight to Oakland and Seattle were placed on another scheduled flight at 7:15 p.m. The grounded aircraft, which had been taken to a remote area of the airfield, was back in service Friday morning, said Greg Witter, manager of media relations for the Seattle-based carrier.

Witter emphasized that the 135-seat plane was searched only as a routine precautionary measure, and that no threat had been received or suspicious object seen.

Witter said the incident began about 5:45 p.m. Thursday, when an Alaska Airlines employee noticed two men standing in a restricted area on the Tarmac near Flight 170, which had arrived from San Jose about 20 minutes earlier. When asked what they were doing, the men told a flight attendant that they worked there, presumably referring to the airport, Witter said.

Moments later, an Alaska Airlines gate attendant approached them as one of the men bolted up the stairs to the waiting aircraft. The man on the ground said his companion was looking for something he had left behind on the plane. Again he indicated that he worked at the airport, but declined to produce identification and finally walked away.

Aboard the plane, where a stewardess was helping some elderly passengers leave, the second man told flight attendants he had left something behind, Witter said. When questioned again, the man bolted out of the plane and into the terminal, eluding pursuing flight attendants.

“It was just enough suspicious activity . . . that airline personnel felt it necessary to check the plane to see if there was a problem,” said Long Beach Police Lt. Greg Whinery, whose department was notified of the incident about 6 p.m.

Long Beach officers searched the plane twice without results, according to Whinery. Then members of the Los Angeles County sheriff’s bomb squad and specially trained dogs borrowed from the Los Angeles Police Department conducted a third uneventful search before the plane was cleared.

The men, described as being of average height and build and in their early to mid-20s, were never found, Whinery said.

Federal Aviation Administration spokeswoman Elly Brekke said the incident was reported to the FAA as required, and that the agency will conduct a routine investigation of airport and airline security procedures.

If lapses in security or violations of federal aviation regulations are found, Brekke said, the airport and the airline could face fines of $10,000.