Nearly 90 homes were damaged many destroyed yet only 10 people of the more than 300 people believed home at the time were hurt, and those injuries were minor, officials said.
Gov. Mitt Romney said the devastation he saw there was shocking.
"The miracle is you have the equivalent of a 2,000-pound bomb going off in a residential neighborhood at night when everybody is home, and no one's dead and no one is seriously injured," Romney said.
Several fires were still burning hours after the explosion. Fire Chief James P. Tutko said it could take days to determine the cause of the blast.
"It looks like a war zone," Tutko said after surveying the area by helicopter. "It's just it's devastating."
The explosion hit the CAI Inc. plant just before 3 a.m. about 20 miles north of Boston. About 20 homes in the surrounding neighborhood probably can't be saved, Tutko said.
Utilities were out across the neighborhood surrounding CAI, and town officials canceled school for the day. Some residents who tried to drive out found their cars were stuck because their garage doors had been blown off the rails.
Residents closest to the blast, including those in a nursing home a facility for the deaf, were evacuated to shelters set up at area schools.
Fred Grenier, 25, was asleep in a second-floor bedroom about 200 yards from the plant when it exploded.
"The windows came caving in. The (air conditioner) fell right on me," Grenier said.
"Everyone was out in the street making sure everybody's all right," he said. "There were windows gone, doors gone, vinyl siding off the houses."
Nancy Chick said the shock bowed her windows inward and pulled her curtains halfway out before the windows returned to their normal position in the frames. Afterward, the curtains hung from their rod and flapped outside her window, even though it was closed.
"I never saw anything like it," said Chick, 66. "All the pressure must have blown it in and then sucked it out."
State Police Maj. Kevin Kelly said he felt the explosion at his home 21 miles away. Some people mistook it for an earthquake, while one caller to WBZ-AM said he looked out his window and saw "the picture of London during the blitz that silhouette."
CAI Inc., a privately owned company, makes solvents and inks and has five to nine employees in Danvers. A person who answered the phone at CAI's Georgetown headquarters Wednesday declined to comment.
Mike Nalipinski, on-scene coordinator for Environmental Protection Agency, said preliminary tests showed a low level of toluene, a solvent, but no significant dangers.
Runoff from water used by fire fighters left a purple sheen on the nearby river, and water tests were being conducted, but Nalipinski said it was not a drinking water supply and the chemical evaporates quickly.
There is an Eastern Propane facility close to the CAI plant, but company spokesman Jeff Taylor said its tanks were secure, though the property suffered some minor damage.