An abandoned consignment of marijuana with a street value of $20m (£10.3m) was found in California when a policeman went to check on a lorry.
A recent study suggests marijuana is the top US cash crop
The vehicle was unlocked and the engine warm, but no-one was in the cab.
The patrolman found plastic-wrapped bundles of the drug in the back of the rental vehicle near Los Angeles after smelling marijuana, AP said.
A spokesman said there were "no suspects and no real leads". Local authorities plan to destroy the drug.
"Somebody's going be in some major trouble for walking away and leaving that quantity sitting on the side of the freeway," said Sgt Telfinues Preszler Jnr of the California Highway Patrol .
"I'm glad I'm not him."
He suggested the engine might have overheated, causing the vehicle to be abandoned along with three tons of marijuana on a slip road in the city of Ontario late on Wednesday.
The drug has been turned over to the San Bernardino County Sheriff's Department, which will seek a court order to destroy it, a spokeswoman said.
A recent report estimates that despite intensive eradication plans, $35bn worth of the marijuana a year is produced in the US, making it the country's top cash crop.