A “suspicious object” which caused a 14-hour closure of part of the M1 was “possibly hydrochloric acid”.
Motorists were left stranded after the 11-mile (18km) stretch between junctions 15 at Northampton and 14 at Milton Keynes was shut from 07:30 BST.
Thames Valley Police said the item was a five litre container in a black bin bag, which contained a “highly corrosive” substance.
The force said it was “keeping an open mind” as to how it got there.
Officers said there was “no evidence to suggest that this is a terrorist-related incident” and are appealing for anyone with information to come forward.
The container was found under a southbound bridge on the road connecting Little Linford and Gayhurst.
Footage showed a bomb disposal robot pulling a bin bag off the hard shoulder, which appeared to contain a yellow liquid.
Insp Scott Long said: “It is still being analysed, however, we do know the substance is a highly corrosive material, possibly hydrochloric acid, which has reacted with another substance, causing it to discharge on to [the road].
“We are keeping an open mind as to how the object was left at the scene.”
The northbound carriageway reopened at 17:00 BST, prior to which many motorists were released from the queues via Newport Pagnell services.
However, the southbound side was shut until 21:00 BST.
Those caught in the delays have criticised the police for poor communication.
Tim Mayer, a Conservative councillor from Coventry stuck in the traffic, said: “I don’t think this is an issue about resource because the reality is you had police standing around stopping traffic from moving and you could go and ask them a question and not get an answer.
“They could have had a statement ready.”
Another motorist said: “I don’t have access to Facebook or Twitter, so for seven hours I’ve had nothing to drink and no information at all.
“I would have liked a policeman to somebody to come up and say, ‘This is the situation and this is what we’re trying to do’.”
A force spokesman said: “We apologise for the inconvenience caused to motorists who were stuck in queues yesterday but your safety, and the safety of our officers and the staff of our partner agencies, is our priority.
“There were officers/staff at the scene and we were regularly updating our website and social media channels.”
In August the same stretch of road was closed for about 12 hours after eight people died in a crash between a minibus and a lorry.
Earlier this month, the M1 between J14 and J15 was closed for 14 hours when a vehicle transporter crossed the central reservation into traffic, killing two people.