The killers of Steven Sotloff were tipped off about the journalist's location by "so-called moderate" rebels in Syria for a cash reward, a spokesman for the family of the slain American journalist said Monday. The family learned of the tip-off from "sources on the ground," Barak Barfi, a friend of Sotloff, said on CNN's Anderson Cooper 360.

The 31-year-old freelance journalist turned up missing during a reporting trip in Syria in August of last year. His videotaped beheading by militants known as the Islamic State of Iraq and Syria followed a similar beheading of American journalist James Foley. The brutal killings have shocked the United States and much of the world and rallied support for an ongoing series of U.S. air strikes against the jihadist group in Iraq.

"Minutes before he was kidnapped, he called me from inside Syria to tell me that he was in," Barfi told Cooper. "Somebody at the border crossing made a phone call to ISIS, and they set up a fake checkpoint with many people," he said. "Steve and his people that he went in with could not escape." The tip-off came from among "the so-called moderate rebels that people want our administration to support," Barfi said. The reward for that information was somewhere between $25,000 and $50,000, he said.

Both President Obama and his Republican critics have at various times called for the use of U.S. military force in support of the "moderate" rebels fighting to overthrow the government of Bashar al-Assad in Syria. The Obama administration has for years advocated Assad's removal, a goal now apparently on hold as the Damascus regime is fighting not only the Syrian Free Army backed by the United States and other nations, but also the same Islamic extremist organization the United States is targeting with air strikes in Iraq. Senators John McCain (R- Ariz.), Lindsey Graham (R-S.C.), and others in Congress who were not long ago calling for military action against the Assad regime are now calling for air strikes against the extremists it is fighting in Syria. A recent study shows the Islamic State has captured caches of arms supplied to Syrian rebels by the United States and allied nations, the Washington Post reported Monday. Captured armaments include small arms and anti-tank weapons, the report said.

Prior to his own execution, Sotloff appeared in the video showing the beheading of Foley. The extremist organization has claimed the beheadings were in retaliation for U.S. bombing of ISIS targets in Iraq. Barfi said there were reports that Sotloff was turned over to his executioners because his name was on a list of people responsible for a previous hospital bombing. "This was false," he said. "Activists spread his name around."

Barfi said his murdered friend "loved the Arab and Islamic world, and he wanted to bring their suffering to the world stage." Sotloff "believed that everybody was created equal, and the people in the Arab and Islamic world weren't terrorists, they were just people like you and me," Barfi said. The relationship between the Sotloff family and the White House has been strained over a request the family made after Sotloff appeared in the video of the Foley beheading, he said. In that video, released August 19, a militant said Sotloff's life depended on what President Barack Obama did next. Another video, showing Sotloff's execution, was released two weeks later.

"Once Steve appeared in that video, the Sotloff family made one simple request of the administration — and they were rebuffed on that," Barfi told CNN. He declined to elaborate on the request, citing the safety of those who are still being held hostage. He also criticized the release of information about an unsuccessful U.S. commando raid this summer that attempted to free Foley, Sotloff, and others held captive by the Islamic extremists.

"We know that the intelligence community and the White House are enmeshed in a larger game of bureaucratic infighting, and Jim and Steve are pawns in that game. And that's not fair," Barfi said. "If there continues to be leaks, the Sotloff family will have to speak out to set the record straight."

"We understand the very real pain the Sotloff family is feeling at this time," National Security Council spokeswoman Caitlin Hayden said. "We condemn the murders of Steven and Jim Foley and we remain committed to bringing the perpetrators of these crimes to justice."

A group of bipartisan senators introduced a bill Monday that would authorize up to $10 million for information leading to the arrest and conviction of anyone involved in the Foley and Sotloff abductions and beheadings, CNN reported. Congress appears unwilling, however, to take a definitive stand on what actions by the president it will support in a yet undeclared and unauthorized war against the Islamic State in Iraq and Syria. Though the Constitution gives to Congress the power to declare war, presidents and the Congress have often found ambiguous undeclared wars more politically expedient. They have not, however, been proven to be less costly, in either lives or money.

Screengrab from ISIS video of Sotloff's "beheading": AP Images