President Barack Obama sent U.S. Special Operations Forces to Syria earlier this year in an attempt to rescue multiple American hostages held captive by the Islamic State of Iraq and Syria (ISIS or ISIL), including American journalist James Foley, who was later brutally murdered by the extremist group.

The operation was unsuccessful, Pentagon spokesman Rear Adm. John Kirby said, because the hostages were not being held at the location where U.S. forces were dispatched.

"The United States attempted a rescue operation recently to free a number of American hostages held in Syria by the Islamic State of Iraq and the Levant. This operation involved air and ground components and was focused on a particular captor network within ISIL. Unfortunately, the mission was not successful because the hostages were not present at the targeted location," Kirby said.

U.S. officials told the Associated Press and other publications that several dozen U.S. troops participated in the operation in eastern Syria. One U.S. soldier was injured while a "number" of ISIS militants were killed, according to the AP.

Officials did not say precisely when the mission took place. Lisa Monaco, the assistant to the president on Homeland Security and counterterrorism operations, would not reveal details of the mission, citing a need to "protect our military's operational capabilities."

The White House intended to keep the operation secret but went public with limited details after "it was clear a number of media outlets were preparing to report" on it.

"We never intended to disclose this operation. An overriding concern for the safety of the hostages and for operational security made it imperative that we preserve as much secrecy as possible," said Caitlin Hayden, a spokeswoman for the White House's National Security Council.

"We only went public today when it was clear a number of media outlets were preparing to report on the operation and that we would have no choice but to acknowledge it."

What is known of the operation is that, as in the raid that killed Osama bin Laden, modified Black Hawk helicopters flown by the elite 160th Special Operations Air Regiment (SOAR) were used, and the team was made up of a "joint force with virtually every service represented," according to an official speaking to The Washington Post.

While the team failed to rescue hostages, troops did seize "unspecified materials" — most likely intelligence documents — from the militants, according to The Post.

"The U.S. Government had what we believed was sufficient intelligence, and when the opportunity presented itself, the President authorized the Department of Defense to move aggressively to recover our citizens. Unfortunately, that mission was ultimately not successful because the hostages were not present," Monaco said.

"Given the need to protect our military’s operational capabilities, we will not be able to reveal the details of this operation. But the President could not be prouder of the U.S. forces who carried out this mission and the dedicated intelligence and diplomatic professionals who supported their efforts. Their effort should serve as another signal to those who would do us harm that the United States will not tolerate the abduction of our people and will spare no effort to secure the safety of our citizens and to hold their captors accountable."

This was not the first time the U.S. military had conducted a raid inside Syria. U.S. Special operations forces conducted a cross-border raid in 2008, killing an Iraqi believed to be helping smuggle weapons, cash, and fighters from Syria into Iraq, according to The New York Times.

ISIS militants seized four additional foreigners in Syria recently, bringing the total number the group is holding to approximately 20, The Guardian reported. The hostages are believed to be held in Raqqa, a town in northern Syria with a heavy ISIS presence.