(CNN) -- Security has been heightened at Sweden's nuclear power plants after explosives were discovered on a vehicle entering a protected nuclear site, authorities said Thursday.
The truck was stopped at the Ringhals nuclear power plant on Wednesday afternoon, the Swedish Radiation Safety Authority said.
The suspicious material was discovered before the vehicle had entered the protected area, it said.
Police are now investigating suspected sabotage, said the plant's owner, Vattenfall.
The "explosive paste" was uncovered by sniffer dogs during a routine security check, the company said in a statement.
"The discovered object could not have induced a serious damage at Ringhals," it said. "Ringhals nuclear power plant is still in operation."
Officers patrolled the site overnight with bomb detection dogs but found nothing else, the company said.
The plant has raised its security level to the second lowest level of 4 as a precaution, the Swedish Radiation Safety Authority said.
The country's two other nuclear power plant, in Forsmark and Oskarshamn, have also boosted their security measures following the discovery, it said.
"It is still assessed that there was no risk of an explosion because the explosive had no detonation device," the statement said.
David Persson, a spokesman for the Swedish Radiation Safety Authority, told CNN he believed it was the first time such an incident had occurred.
The State Forensic Laboratory confirmed the substance was an explosive paste Thursday morning, Vattenfall said.
Sweden has 10 reactors at its three nuclear plants.