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Obama calls bomb blast at Boston Marathon ‘an act of terrorism;’ explosives were hidden in pressure cookers

President Obama today called the twin bombings of the Boston Marathon “an act of terrorism.” (Getty Images)

The bloodthirsty terrorist who bombed yesterday’s Boston Marathon probably carried out heinous attack by loading explosives in kitchen pressure cookers and concealing them in heavy nylon bags, authorities revealed.

Investigators have found traces of 6-liter pressure cookers, which contained shards of metal, nails and ball bearings for the crude-but-deadly bombs that were likely set off by a timer, law enforcement sources told The Post.

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Authorities have also found a circuit board believed to have been used to trigger the bombs, sources told The Boston Globe.

Richard DesLauriers, the FBI’s special agent in charge of the agency’s Boston office, declined to discuss specifics, but confirmed today that investigators believe pressure cookers were used.

“Possibly a pressure cooker … black nylon bags, possibly at both [blast] sites,” DesLauriers said.

“We’re postulating that they needed to be heavy bags to carry the explosive devices inside of them. They would not be light bags.”

The Department of Homeland Security today advised police agencies to be on the lookout for any pressure cookers on the street and deem them suspicious.

“Terrorists can exploit the innocuous appearance of easily transportable items such as pressure cookers to conceal IED components,” according to the Homeland Security memo, obtained by The Post. “Placed carefully, such devices provide little or no indication of an impending attack.”

Homeland Security issued a similar warning to local cops in 2004, following attacks in India using a pressure cooker.

The unsuccessful May 2010 Times Square bomber fashioned one of his three explosives from a home kitchen pressure cooker, filled with M-88s and a metal rifle case.

President Obama, who today labeled the attacks “an act of terrorism,” will come to Boston on Thursday for an interfaith memorial service, Massachusetts Gov. Deval Patrick announced.

Patrick urged Americas no to jump to any conclusions about the individual or group responsible for yesterday’s terror attack.

“This community will recover and will heal, if we turn to each other rather than on each other,” Patrick said.

“One of things that we’ll emphasize at the interfaith service … is that we are one community, as the mayor said, we are all in this together.”

President Obama praised Americans who “refuse to be terrorized.”

“This was a heinous and cowardly act,” Obama told the nation, less than 24 hours since two bombs ripped apart Boston streets. “The FBI is investigating it as an act of terrorism.”

With details still scarce about the twin blasts at the marathon’s finish line, Obama said it’s not clear if an organization or lone wolf did this deadly deed.

“What we don’t yet know, however, is who carried out this attack or why,” the president said. “Whether it was planned and executed by a terrorist organization – foreign or domestic – or was the act of malevolent individuals. That’s what we don’t yet know. Clearly we’re at the beginning of our investigation.”

The president said he was impressed by first responders and average citizens who rushed to help the injured.

“The American people refuse to be terrorized because what the world saw yesterday … were stories of heroism and kindness and generosity and love,” Obama said.

“Exhausted runners who kept running to the nearest hospital to give blood and those who stayed to tend to the wounded, some tearing off their own clothes to make tourniquets.”

The twin blasts killed at least three people and injured 176 — including 17 in critical condition, authorities said today.

More than half of the marathon’s runners had crossed the finish line when the explosives went off.

Bomb-detecting cops had wept the finish-line area twice yesterday morning, once early in the morning and again an hour before the first runners crossed, Boston police commissioner Ed Davis said.

“Those two EOD sweeps did not turn up any evidence,” Davis said.

But the city’s top cop said there was no way to prevent an attacker from coming and going, and perhaps planting explosives after police had swept the area.

“People can come and go and bring items in and out,” he said.

Special Agent DesLauriers vowed to go to the “ends of the earth” to hunt down the terrorists.

“This will be a worldwide investigation,” he said. “We will go to the ends of the earth to identify the subject or subjects who are responsible for this despicable crime and we will do everything we can to bring them to justice.”

Many of the injured suffered small wounds in their legs, maimed by what looked like ball bearings or BBs, according to ER doctors.

Injures ranged from cut knees to blown-off limbs.

“I was close enough to see some of the carnage — all [injuries to the ] lower extremities and people who had their limbs severed,” Boston City Council President Stephen Murphy told The Post today.

“One woman I saw lost half her leg. It was a surreal scene.”

Murphy was about 50 feet away from the finish line and would have been a lot closer to the bombing — but he had stopped walking to let a TV reporter finish a shot.

“It was like a big fireball and explosion — and then another one 15 seconds later,” he said. “We were instantly scattering.”

With Post Wire Services and additional reporting by Dan Mangan