U.S. officials have questioned suspected Islamic State militants who Syrian forces believe have links to a suicide attack that killed four Americans in January, an American official said Tuesday.

The official, speaking on the condition of anonymity because he was not authorized to comment publicly, said the prisoners were being held by the U.S.-backed Syrian Democratic Forces (SDF), a Kurdish-dominated group that has been the chief on-the-ground American partner in Syria.

The official spoke hours after the SDF announced that it had captured suspects in the attack, which targeted Americans posted to Syria’s northern city of Manbij in order to conduct counterterrorism and intelligence operations. It was the single deadliest incident involving U.S. government personnel in the war against the Islamic State.

“A group of suspects believed to be involved in January 16 Manbij bombing that killed several U.S. and SDF servicemen were captured following technical surveillance by our forces,” SDF press official Mustafa Bali said on Twitter.

The Americans killed in the attack were Jonathan Farmer, a Green Beret; Shannon Kent, a Navy cryptologist; Scott Wirtz, a civilian intelligence officer, and Ghadir Taher, a contractor. At least two SDF personnel and eight civilians were also killed, local officials have said.

The news of the suspects’ detention was first reported by Reuters. The official said as many as five suspects were questioned.

It was not immediately clear whether the Trump administration, if the detainees’ suspected links to the attack bear out, would seek to bring the suspects to the United States for prosecution or further detention. The administration has said it would be open to putting new terrorism suspects in the military prison at Guantanamo Bay, Cuba, but that move could pose legal and logistical challenges.

Officials have also considered using federal courts to try Islamic State suspects believed to be involved in the death of Americans.

The detention of those and other Islamic State suspects takes place as the Trump administration prepares to reduce its troop footprint in Syria. While President Trump has pushed for ending U.S. military involvement in Syria, officials have persuaded him to allow a small residual force to remain.

Liz Sly in Beirut and Ellen Nakashima in Washington contributed to this report.