Spanish police have arrested a man who was stocking 755 kg of dynamite in his garage - nearly four times the amount accumulated by the Madrid train bombers.
He also had 750 kg of ammonium nitrate, a fertiliser that can also be used to make bombs, as it was in the 1995 Oklahoma City bombing in the United States.
Investigators suspect the man could have saved the dynamite over the past 13 years on sites where he worked as an explosives expert, a police spokesman said.
His motive for hoarding the stash was unclear.
"He had no criminal record and was not linked to radical or terrorist circles or anything like that," the spokesman said.
The dynamite was of the Goma 2 Eco type, the same used by the Madrid train bombers in attacks on March 11th, 2004 that killed 191 people and injured 1,900.
The explosives used in the train bombings had been stolen from a mine in northern Spain, prompting the government to reinforce measures to keep track of explosives.
The train bombers had about 200 kg of Goma 2 Eco. They used it for the March 11th attacks as well as an attempt to blow up a high-speed train two weeks later and a suicide explosion in which seven of the main suspects and one policeman died.
The man had a bag on him containing 33 packages of Goma 2 Eco when he was arrested in the northeastern region of Catalonia. At his home, he was also stocking 5,000 detonators and 500 metres of fuse.