A suspected component of a bomb used in the deadly Boston marathon attack was found by accident on the roof of a city hotel, the owner told AFP.

The pressure cooker lid was found atop the Charlesmark Hotel, close to the marathon finish line where the bombs were placed, said owner Mark Hagopian.

Investigators have said they believe the bombs, which killed three and injured around 180, were packed in pressure cookers and the lid could provide valuable evidence in the hunt for the attackers.

"The guest who found it should not have been on the roof," Hagopian said. "Normally the door is locked."

The man, from Portland, Oregon, had already been told to leave the rooftop, six floors above street level, after police spotted him there on the morning of the marathon, Hagopian added.

"Police told us to clear it," he said.

The guest, whose girlfriend was running in the race, returned however. And just after the bombs exploded he found what he thought was the hub of a car wheel and handed it to police.

Debris from the bomb was spread over a wide area.

"I don't know how he got on the roof, so the cooker lid was found by luck," said the owner of the hotel which is mainly used by runners during the marathon week.