Police are investigating the potential source of a major oil spill in a West Yorkshire river.
About 5,000 gallons of diesel, worth thousands of pounds, leaked into the River Aire in Leeds on Saturday night.
The Environment Agency said the oil had covered an eight mile (13km) stretch from bank to bank between Leeds and Castleford.
It is thought that the spill was caused by thieves attempting to steal diesel from an oil tank.
West Yorkshire Police confirmed that officers were investigating the theft of a large amount of diesel from a business on Millshaw Park Industrial Estate in the city.
The Environment Agency said it was alerted to oil flowing into the River Aire at Water Lane between 17:40 and 19:40 BST on Saturday.
Contractors and staff from the agency and the Canal and Rivers Trust used absorbent booms and pads to soak up as much of the oil as possible.
The agency said local wildlife was not thought to be at risk.
"The sheer quantity of oil in the fast-moving waters of the river and canal system means that a large part of it will be flushed downstream into the Humber," an Environment Agency statement said.
"By this time it will be very dispersed and the risk to the environment will be greatly reduced."
Police said inquiries were continuing.