Bombs left at a Southern California social services facility by the gun-wielding radical Muslim couple who killed 14 and wounded 21 were set to go off when first responders arrived, Fox News learned on Monday, in a vicious strategy often seen in the Middle East.

None of the pipe bombs left at the Inland Regional Center in San Bernardino by Syed Farook and Tashfeen Malik in Wednesday's attack detonated, but the technique has investigators very concerned, sources told Fox News. 

"This was meant to kill more, but also scare other future responders to attacks," a source with inside knowledge of the investigation said. "This was meant to get into the minds of medics and officers who are arriving first on scene."

It remains unclear why the bombs did not detonate. It could have been water from the sprinklers or a malfunction with remote control devices, according to investigators.

The couple burst into the facility, where Farook's employer, the county board of health, was holding a holiday party. Blasting away with assault rifles, they gunned down victims and fled within minutes in a dark SUV. Several hours later, police apparently suspecting their involvement and staking out their Redlands home saw a vehicle matching that description. The ensuing chase ended in a hail of bullets on San Bernardino Avenue, the city main road, just two miles from the center.

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In addition to explosives found in the SUV, authorities discovered and detonated three pipe bombs late Wednesday at the Inland Regional Center, the complex where the initial shooting took place about 60 miles east of Los Angeles. Another source said investigators discovered a dozen pipe bombs in the house, as well as small explosives strapped to remote-controlled cars - a signature of terrorist groups including Al Qaeda, according to counter-terrorism experts. Police also found thousands of .223-caliber and 9mm rounds at the home.

The FBI says the California shooters had participated in target practice, including once within days of the attack.

David Bowdich, assistant director of the FBI's Los Angeles office, said at a news conference Monday that the couple took part in target practice at ranges in the Los Angeles area. He said the bureau believes both were radicalized and had been "for some time," but that the bureau doesn't know when or how they were radicalized.

The FBI has said it's investigating the shooting at a holiday gathering of Farook's co-workers as an act of terrorism.

Bowdich told reporters that investigators found 19 pipes in the couple's home in Redlands, California, that could be turned into bombs with all the right components.

Federal authorities also say they're trying to determine how two assault rifles used in the attack got from a buyer to the shooters.

John D'Angelo, assistant special agent with the Bureau of Alcohol, Tobacco, Firearms and Explosives, said Farook bought three guns that have been recovered, including two pistols that he and his wife used in the shooting.

D'Angelo said at a news conference Monday that Enrique Marquez purchased two others, the assault rifles. D'Angelo says federal authorities are investigating how those weapons got from Marquez to Farook and Farook's wife, Tashfeen Malik, who helped carry out the attack that left 14 dead.

Marquez hasn't been charged with a crime, and it's unclear if he did anything illegal.

D'Angelo says the weapons are undergoing forensic tests and they all were legally purchased in California between 2007 and 2012.

Fox News Channel's Adam Housley  and the Associated Press contributed to this report.