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Underscoring the tension at airports since last week’s terrorist attacks, a couple and another man were detained and later released Monday after they locked themselves in lavatories on a morning flight from Austin, Texas, to Chicago.

In another incident early Monday, a man allegedly attempted to pass through security at O’Hare International Airport with a loaded gun in a carry-on bag.

When American Airlines Flight 528 from Austin landed safely at O’Hare International Airport at 10:33 a.m., Chicago police and FBI agents surrounded the plane, then boarded and removed the three people in handcuffs.

They were detained because they ignored the pilot’s instructions to return to their seats as the plane made its approach, Chicago police and an FBI spokesman said.

In a separate incident, another man went to a different lavatory and would not return to his seat.

The man and the couple did not know each other, FBI spokesman Ross Rice said.

Bomb-sniffing dogs checked carry-on bags, and police and FBI agents checked all luggage, on the plane, police said. The 55 other passengers on board were briefly interviewed by FBI agents.

“What would have been stupid a week ago now becomes an incident,” Chicago police spokesman Patrick Camden said.

The airport remained open during the incident. One passenger, Joe Namee, 31, of Austin, said police acted properly.

“They got the people they wanted to get and they took them off. I think we were alarmed, but I don’t think anybody was really nervous or scared because it happened pretty quick,” Namee said.

Russell Jones, 56, was at the Terminal 1 checkpoint at 7 a.m. when an X-ray of his bag revealed a loaded small-caliber handgun, said Sgt. Robert Cargie, a Chicago police spokesman.

Jones, of the 1800 block of North Mohawk Street, Chicago, was arrested and charged with unlawful use or possession of a weapon, Cargie said. Jones didn’t say why he was carrying the gun, Cargie said.