The Islamic terrorist couple who slaughtered 14 people in Southern California and wounded 21 others last week would have killed more had the bombs they left at the site of the massacre exploded when the first responders arrived, law enforcement sources revealed.

Fortunately, the pipe bombs left by Syed Rizwan Farook and his wife Tashfeen Malik at the Inland Regional Center in San Bernardino did not detonate, the sources told Fox News.

The technique used by the couple raised new concerns since this vicious strategy, often seen in the Middle East, could signal more devastating terrorist attacks in the U.S., sources said.

Meanwhile, investigators are pointing to Pakistan as the likely place where the Muslim couple were radicalised, Fox News learned. Pakistan is also the country that once harboured Al Qaeda founder and 9/11 mastermind Osama bin Laden.

Investigators are focusing on Pakistani-born Tashfeen Malik, 29, whom U.S.-born Syed Rizwan Farook, 28, brought to the U.S. just last year on a fiancée visa after they met online. They later got married.

U.S. Attorney General Loretta Lynch said investigators have already interviewed hundreds of people and are working with Pakistan and other foreign governments to explore the Pakistan connection in the terrorist attack. Pakistan's interior minister also announced that his country had launched its own investigation.

Although the U.S. and Pakistan are considered as allies in the war on terror, their ties have remained uneasy because of a sense of mistrust, officials said.

Bin Laden was believed to have lived for years in a tightly guarded compound in Pakistan, possibly with the knowledge of government authorities, prior to the May 2011 Navy SEAL raid that led to his death.

Sources said the U.S. is putting heavy pressure on Pakistan to cooperate fully with U.S. authorities in investigating the deadliest terror attack on the American homeland since 9/11.

Malik was said to have been born to a wealthy family in Pakistan's southern Punjab province, moved to Saudi Arabia as a child and returned to Pakistan to study pharmacology in 2007.

Investigators are also checking on the people in the U.S. who may have aided the terrorist couple. They raised the question on how Farook—with his meagre $51,000-per-year salary as a county restaurant inspector—was able to purchase an arsenal of weapons including handguns, rifles, thousands of rounds of ammunition, bomb-making supplies and equipment.

It is doubtful that the couple could have financed their terror activity on Farook's salary, said House Homeland Security Committee Chairman Michael McCaul, R-Texas.

He said investigators are busy trying to find out whether the couple had financial help from terrorists either in the U.S. or overseas.