Airport security: Four passengers removed from flight in Baltimore

Law enforcement is questioning three men and one woman who were removed from a Chicago-bound flight from Baltimore Tuesday morning after a fellow passenger alerted flight crew to suspicious activity as the plane taxied toward the runway.

Spirit Airlines Flight 969 returned to the gate at Baltimore-Washington International Thurgood Marshall Airport, where all aboard were temporarily deplaned, according to ABC 7's Suzanne Kennedy, a passenger on the flight. 

Crew asked police to remove four passengers, who are being interviewed. Maryland Transportation Authority representative Sgt. Jonathan Green told the Baltimore Sun that none of them were under arrest as of 9:30 Tuesday morning.

Reuters reports that a government source said a female passenger who appeared to be Muslim was watching a video on her phone that another passenger thought was from Islamic State. The militant group has claimed responsibility for the attacks in Paris on Friday that killed 129 people.

A woman got up and went to the back of the plane with a small child as the plane was preparing for takeoff, said WJLA, an ABC affiliate. First Sergeant Jonathan Green, a spokesman for the Maryland Transportation Authority police, said one of the four had been questioned and released.

Spirit Airline said that the flight, originally scheduled to depart at 6 a.m., had been cleared by the Transportation Security Administration and took off after 9 a.m..

On Sunday, two passengers were removed from an American Airlines flight at nearby Ronald Reagan Washington National Airport. In that case, the pair were questioned, then put on another flight; no charges have been filed.

The crash of a Russian charter plane in Sinai, Egypt last month, which Russian officials now say was due to a homemade bomb explosion, raised alarm in US airports, which moved to tighten security. Worries over airport safety escalated after last week's terrorist attacks in Paris, which killed more than 120 people, and an Islamic State video released on Monday which threatens similar violence in Washington, DC. 

In Atlanta, the busiest airport in the world, security increased for both passengers and 40,000 staff, stepping up employee screening and cutting the number of access points to secure areas. 

Atlanta passenger Blake Alford says he accidentally flew to Chicago with a loaded semi-automatic pistol in his carryon, which airport security overlooked.

"People need to know TSA needs to tighten up," Mr. Alford told CBS 46. "They'll take toothpaste. They'll make people get out of wheelchairs. They'll make me take off my belt buckle and my shoes. How did my gun go through?"

This report contains material from the Associated Press.