(CNN) US-backed forces in Syria have captured a handful of ISIS fighters linked to the January suicide attack in Manbij, Syria, that killed four Americans, according to a US defense official.

A spokesman for the SDF, Mustafa Bali, confirmed the capture in a tweet, writing that it occurred "following technical surveillance by our forces."

The US defense official said up to five ISIS members who were involved in the planning and execution of the attack are being detained by the Syrian Democratic Forces. The US has also questioned the militants, who were captured in Manbij.

A second defense official says a direct link to the Manbij attack has not yet been fully proven but the belief is the fighters are linked to terror operations in the area so they may "likely" have ties to the attack. The investigation is continuing to see if there are more senior ISIS operatives who may have played a role.

A group of suspects believed to be involved in January 16 Manbij bombing that killed several US and SDF servicemen were captured following technical surveillance by our forces. The outcome of the ongoing investigation will be shared at a later time. — Mustafa Bali (@mustefabali) March 19, 2019

Their capture was first reported by Reuters.

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