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The mother of American journalist James Foley, who was beheaded by Islamic State militants last year, demanded proof on Tuesday that U.S. policy not to negotiate with terrorists is saving American lives and decreasing the rate the U.S. citizens are being captured.

"I recognize that it is complex because we certainly don't want to fund terrorists," Diane Foley told a House subcommittee. "But is it wise to not even engage these people? ... Then we don't know what's going on. Then we don't know what they want. We don't know who they are. I just think we need to be a lot shrewder."

James Foley, 40, went to Syria in 2012 to cover escalating violence there. He was captured in November 2012 when the car he was riding in was stopped by four militants in a battle zone that Sunni rebel fighters and government forces were trying to control. He was beheaded in a video released by the militant group in August 2014.

After Foley disappeared while contributing video for Agence France-Presse and the media company GlobalPost, his parents became fierce advocates for him and all others kidnapped in war zones. Diane Foley said that during her son's captivity, the U.S. policy not to pay ransom was interpreted to mean "no negotiations, no engagement with his captors."

"I am told our strict adherence to this policy saves lives by decreasing the rate of capture of Americans, but no one has been able to show me the research behind our hostage policy," she said. "In fact, it would seem that Americans are becoming targets at an alarming rate."

She said that during one month that IS captors reached out to negotiate for Foley's release, the U.S. government refused to directly engage with them "leaving us alone as parents, to try to negotiate for our son's freedom."

She said that 18 months after her son was kidnapped, her family and the three other families of hostages held with him in Syria were threatened three times by a member of the National Security Council with government prosecution if "we attempted to raise a ransom to free our loved ones."

European governments routinely pay ransom to win the release of hostages. However, President Barack Obama and his predecessors have argued that policy provides terrorists with funds to fuel dangerous activities and puts Americans at greater risk of kidnapping. Foley of Rochester, New Hampshire, said families, by themselves, often cannot amass the money needed to match the sizeable ransom payments demanded by terrorists.

In June, after a six-month review of the U.S. hostage policy, Obama conceded that the U.S. government had let down the families of Americans held hostage by terrorists and promised they would never face criminal prosecution if they wanted to pay ransoms to their loved ones' captors. Obama said then — for the first time — that U.S. government officials also can communicate directly with terrorists and help families negotiate for the release of hostages.

By clearing the way for a family to pay ransom without fear of criminal charges, Obama essentially said families could take actions the U.S. government has long said put Americans at risk. He said the government, however, would continue to abide by prohibitions on paying ransoms or making other concessions to terrorists.

Rep. Ted Poe, R-Texas, chairman of the subcommittee, said IS made more than $1 billion in 2014 from several sources, making the group one of the best-financed terrorist groups in history. "Three ongoing and increasing sources of funding for the group are kidnapped for ransom, antiquities trafficking and private donations," Poe said.

John Cassara, a former intelligence officer and special agent at the Treasury Department, testified that in recent years, terrorist and associated organized criminal organizations have turned to kidnapping as an easy and lucrative source of funding.