Almost 50 people were evacuated from a coach on a motorway after armed police closed the M6 Toll in Staffordshire on Thursday morning.
The M6 Toll road near Lichfield was closed in both directions after allegations were made against one of the 48 passengers on board the Megabus coach from Preston to London, the coach firm said.
Staffordshire Police, who were called to the scene at 8.20am, said there was an ongoing incident at Weeford, near Lichfield, but could provide no further details.
When the single-decker coach left the scene, escorted by a number of police vehicles, shortly after 12.35pm, a spokesman for Staffordshire Police said: "This is not being treated as a terror-related incident."
A spokeswoman for Megabus, which is operated by Stagecoach, said all the passengers were safe and well.
"We are assisting police with their inquiries into an allegation made against a passenger who was travelling on the 05.10 Preston to London service," she said.
"There were 48 people booked to travel on board the service, which was due to arrive at Victoria Coach Station in London at 10.55am.
"The service is stopped at the M6 toll plaza.
"Police have confirmed that all passengers are safe and well and they have been transferred to a substitute vehicle.
"We are making arrangements to provide onward travel for passengers as soon as we receive clearance from the police."
Reports suggested a man was seen pouring liquid into a smoking bag on board the coach.
Armed police officers could be seen positioned next to the coach on the south-bound carriageway, as passengers were led off the vehicle one by one.
A number of emergency service vehicles were parked at the toll plaza, about 200 metres from the coach. Seven police vans arrived at the scene shortly before 11am.
It is understood the coach, which was heading south on the M6 Toll when it stopped, pulled over on to the hard shoulder of the motorway of its own volition.
The road was closed in both directions between junctions JT3 and JT4, near the A38 and A5, the Highways Agency said.
Several people, including at least one young boy, were seen sitting on the motorway in a cordoned-off square, several hundred metres from where the coach stopped.
Police surrounded the cordoned-off area, which spread across one lane of the toll road. Police dogs and their handlers were also seen nearby.
Several other cordoned off areas could also be seen, along with what appeared to be bright yellow tents on the other side of the toll plaza.
Eyewitness Nick Jones, who was stopped on the motorway for more than an hour and a half, told BBC News that police warned him to stay in his car, keep his windows closed and not to use air conditioning.
"I was beginning to feel a little uneasy," he said.
"I was beginning to look around for an escape route."
Military personnel were at the scene with a truck marked Royal Logistic Corps Bomb Disposal.
A spokeswoman for the Ministry of Defence said: "The Ministry of Defence is providing assistance to the police at their request under routine military procedure."
Police were searching the coach passengers one by one.
Traffic information company INRIX said traffic avoiding the M6 toll closure were using the M6 proper and being caught in delays of up to two and a half hours on the southbound carriageway because of an accident around junction 7 at Great Barr, north of Birmingham.
Two lanes were closed because of that accident, with delays as far back as junction 13 south of Stafford.
Drivers wanting to avoid the M6 queues were advised to take either the A449 and travel through Wolverhampton to rejoin the M6 south of junction 7, or use the A34 and head south from Stafford.