ERBIL, Iraq — American-backed forces in northern Syria have captured a group suspected of organizing a bomb attack that killed four Americans and a number of allied Syrian militia fighters in January, a spokesman for the local forces said on Tuesday.
The spokesman, Mustafa Bali, wrote on Twitter that his group, the Syrian Democratic Forces, had used “technical surveillance” to find and arrest the suspects.
The bombing, on Jan. 16, was the deadliest attack on Americans in Syria since Washington began a military campaign against the jihadists of the Islamic State in 2014, and it occurred in an area the United States had considered safe. Identifying those involved could help explain how the jihadists were able to strike far from their last stronghold.
The arrests were first reported by Reuters. A United States military official confirmed that Syrian allies had captured a small number of people linked to the attack in recent days but provided no further detail.
Col. Sean Ryan, a spokesman for the American-led military coalition against the Islamic State, declined to comment, citing in an email a continuing investigation.
In the attack, a suicide bomber entered a popular restaurant in the Syrian town of Manbij where American military personnel had stopped during a patrol to eat with their local Syrian allies.
The Americans killed were a military linguist who worked with the National Security Agency, a member of the Army Special Forces, a former Navy SEAL who did intelligence work for a Pentagon agency and an Arabic interpreter who worked for a private defense contractor.
At least 11 Syrians were also killed, most of them members of the Syrian Democratic Forces, a Kurdish-led militia that has been working with the United States to fight the Islamic State.
Mr. Bali did not say how many suspects there were, or when or how they had been arrested.
The arrests come at a critical juncture for the future of the United States presence in eastern Syria.
President Trump has said that he wants to withdraw the roughly 2,000 American troops posted there, although that plan has been repeatedly revised, leaving American allies and even many United States officials confused about what Washington’s policy is. Recently, the Trump administration has said that it is planning to leave 400 troops in Syria to continue working with American allies on the ground and to stop Iran and its allies from making inroads in the area.
It remains to be seen whether the American withdrawal will accelerate after the defeat of the Islamic State’s last patch of territory. On Tuesday, officials from the Syrian Democratic Forces said the jihadists’ territory had been reduced to a small patch of land along the Euphrates River near the village of Baghuz.
But the battle there has lasted much longer than predicted, and tens of thousands of fighters and civilians have surrendered as the jihadists’ territory has dwindled. Most of the men are in prison and most of the women and children are in overcrowded refugee camps.