Britain scrambled fighter jets Friday to intercept a commercial airliner carrying more than 300 people from Pakistan, diverting it to an isolated runway at an airport on the outskirts of London and arresting two British passengers who allegedly threatened to destroy the plane.

A British security official who requested anonymity said the situation involving the Pakistan International Airlines flight did not appear terror-related, though police were still investigating, but the incident further rattled the U.K. just days after a soldier was killed on a London street in a suspected terror attack.

A Pakistani official briefed by British police and PIA security on the investigation said the two suspects, speaking Urdu, allegedly threatened to “destroy the plane” after an argument with crew, then went on to say they were only joking.

Flight P709 was traveling from Lahore, Pakistan, to Manchester Airport when it was diverted by the fighter jets to Stansted Airport. The U.K. Ministry of Defense confirmed that Typhoon jets were launched to investigate an incident involving a civilian aircraft but gave no further details.

Deployment of warplanes is a standard procedure, according to defense ministry officials, when pilots using emergency codes raise the alarm. The Typhoons took off from an air base at Coningsby in Lincolnshire.

An official who spoke in return for anonymity under ministry rules said such deployments “happen more often than we would think. It’s standard procedure in situations when, for example, contact with the pilots and the plane is lost.”

Passenger Nauman Rizvi told Pakistan’s GEO TV that two men who had tried to move toward the cockpit during the flight were handcuffed and arrested once the plane landed. Rizvi said that after the men were taken away, the flight crew told passengers there had been a terrorist threat and that the pilot had raised an alarm.

Essex Police said they were notified at 1:20 p.m. British time that a threat had been made to an aircraft. The force said that after the Boeing 777 landed at 2:15 p.m., armed officers entered it and arrested two British nationals, aged 30 and 41, on suspicion of endangering the aircraft.

The suspects were taken to a police station where they were questioned, the police said in a statement. The plane will be examined by forensic specialists but no suspicious items have been recovered so far, police added.

“This incident is being treated as a criminal offense,” the police statement said, in another indication it was not being seen as a terror case.

Mashood Tajwar, a spokesman for PIA, said 297 passengers and 11 crew members were on the plane. By late afternoon Friday, passengers had disembarked from the plane and were being interviewed, according to Mark Davison, a spokesman for Stansted Airport.

One passenger told Britain’s Sky News that the two men had attempted to get into the cockpit.

“The cabin crew informed us that basically they tried to come into the cockpit a few times and because they had been asked not to do that they got into a bit of an argument with the crew and made a few threats,” Umari Nauman said.

The plane remains isolated on the north side of the airport, with flights proceeding as usual. An airport spokesman said the airline was making arrangements to transport passengers to Manchester.