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.