The Boston Marathon bombers were part of a secret terrorist cell plotting to unleash a massive campaign of death and destruction on major American cities including New York and Washington, D.C., an ENQUIRER investigation reveals.

The tragedy that stunned America left four dead and 183 injured – but the devastation could have been far worse because the two brothers accused in the attack were allegedly part of an army of hell-bent jihadist warriors.

“Clearly these two brothers were sleeper agents just waiting for the word to attack U.S. citizens,” espionage expert Paul Huebl told The ENQUIRER.

“We know they got training about how to build these bombs and where to leave them to do the most possible damage.”

Huebl, a former Chicago cop who investigated the Baader-Meinhof terror cell when he was in the army in Germany, added: “These guys were obviously reporting to a highly trained jihadist at a terrorist camp in America.

“The younger one was wearing Louis Vuitton shoes that cost $900 and Tamerlan drove a Mercedes, yet neither of them had a job. Who was paying them? Who paid for Tamerlan to go to Russia for six months of terror training? And who were his handlers here in the states? It takes a lot of money to buy all the firepower these guys had.”

And yet, Tamerlan was collecting welfare checks from the very county he despised.

In the wake of the Boston attack, more than 1,000 government agents fanned out to find a terrorist “sleeper cell” they believe is living secretly here in the U.S. There are a dozen members out there somewhere and the Tsarnaev brothers are thought to be members of the kill squad.

The U.S.-hating group plans to strike fear into the hearts of Americans, said sources.

There are indications the U.S. Government is hiding other de­tails about the Boston bombings to minimize fear and anger from citizens. A Justice Department source confirmed Tamerlan fell completely off the FBI’s radar, even though he was questioned before – and after – a 2012 trip to Russia.

Authorities also have information that an “Al-Qaeda secret cell operating in the U.S. was planning something big,” plots that include terror­ist attacks on several cities.

MOVING SWIFTLY, NOW THE FBI PICKED UP TWO men for questioning about 60 miles from Boston shortly after the blazing April 19 gun battle with alleged bombers Tamerlan Tsarnaev, 26, and his 19-year-old brother Dzhokhar.

The men were friends of the younger Tsarnaev brother and drove a BMW with a fake front plate that read “Terrorista # 1.”

Sources told The ENQUIRER that had the bloodthirsty brothers not been stopped, they would have proceeded to attack other East Coast cities, including our nation’s capital and New York, the home of our financial system.

“In New York, they likely would have targeted a crowded street in or around Times Square or a busy place like Grand Central (train) station, or even Wall Street itself,” declared a Justice Department insider. “In D.C., they might have set off bombs at Amtrak’s Union Station or outside the White House. These guys were brazen – they thought nothing could stop them!”

After the deadly shoot-out, Boston police dis­covered that the two brothers had an incredible arsenal of weapons with them – including multiple handguns, a rifle, 250 rounds of ammunition and at least six homemade bombs.

Boston Police Commissioner Ed Davis declared: “We have reason to believe, based upon the evidence that was found at that scene – the explosions, the explosive ordnance that was unexploded and the firepower they had – that they were going to attack other individuals.”

Their attack plan to unleash another bomb in New York’s Times Square has been confirmed by law enforcement authorities.

Before coming to the United States nearly a decade ago, the brothers had lived in Russia’s strife-torn Dagestan Republic and Kyrgyzstan.

Concerned about Tamerlan’s interest in Islamic extremists in his native country – and his planned trip to his homeland – the Russian government asked the FBI to interview him two years ago. U.S. agents determined Tamerlan was not a threat.

The young man, however, was upset about being quizzed and “wanted revenge,” the State Depart­ment official told The ENQUIRER.

When Tamerlan returned to the Dagestan region for six months in 2012, it is believed that he trained to become a killing machine.

The aspiring terrorist secretly traveled to an Al-Qaeda-run camp in a remote forest location near Dagestan, where fighters learn hand-to-hand combat, marksmanship and how to make bombs.

“Tamerlan was trained by terrorists loyal to Osama bin Laden and connected to self-proclaimed rebel leader Doku Umarov, suspected of master­minding a 2010 attack on the Moscow subway that killed at least 40,” said the Justice Depart­ment source.

Dmitry Pavlov of Russia’s National Anti­terrorism Committee told The ENQUIRER it’s also likely Tamerlan trained at a recently raided radical camp in the village of Gimry, Russia. “Their laboratory was organized right in the village mosque, and some of their notebooks vividly described how to make a bomb.”

There were also timing devices present, said Pavlov, “a type that may have been used in Boston.”

When Tamerlan returned from Russia to the U.S. in July 2012, he visited New York before heading to Boston.

“While in New York, he probably scouted locations for terror attacks,” said the Justice Department source.

As an aspiring boxer, Tamerlan also traveled to other metropolitan areas, including Salt Lake City, Utah. “The boxing may have been a cover to allow him to scope out potential targets,” said the source.

Espionage expert HUEBL noted that “it takes a lot of money to buy all the firepower these guys had. Authorities will do a forensic review of their phones and In­ternet use to figure out who were their boss­es. A lot of heads will roll before this is over.”

Investigators say that over time Tamerlan brainwashed his younger brother Dzhokhar with anti-American rhetoric and that they came up with the plot to attack the Boston Marathon.

“Why DID they kill? Tamerlan snapped after a series of events in which he realized he couldn’t cut it in the greatest country on earth – and he convinced his younger brother to go along with him on a path to destruction,” said a Justice Department source. “He wanted revenge against America because he was a failure.

“His mom was arrested in Boston for shoplifting. His marriage was broken up. He had a major chip on his shoulder and turned it against America. He was the ultimate outsider and he hated it.”

The brothers’ death-dealing plot meshed perfectly with plans to attack U.S. targets recovered from Osama bin Laden’s Pakistan compound.

To spread his jihadist message, Tamerlan turned to social media and created a You­Tube channel with links to terror-related videos. His channel used the online address “muazseyfullah,” which combines the names of two prominent Islamist militant leaders in Russia. Part of the word – “Seyfullah” – also translates as “Sword of Allah.”