A welder entering the plant was caught with a small amount of what authorities characterized as a highly explosive substance, Kalmar police spokesperson Sven-Erik Karlsson told the Swedish News Agency TT.

"According to the information we've received, the substance is known by the abbreviation TATP and is highly explosive," Karlsson said. "We understand that it isn't something he needed for the job."

TATP is the substance police said they believe was used in the July 2005 terrorist attacks in London.

Police cordoned off an area near the plant's entrance after the explosives were found. Karlsson said the plant's operations were not affected.