Baghdad ? Four members of an Iranian-backed Shiite cell confessed to bombing a public market in central Baghdad, a U.S. spokesman said Saturday.

He also blamed Shiites for recent attacks on U.S. bases, raising fears that a three-month truce by the most feared Shiite militia may be at an end.

The blast Friday in the al-Ghazl pet market killed at least 15 people, wounded 56 and shattered a growing sense of public confidence that has emerged following a sharp decline in the bombings and shootings that once rattled the Iraqi capital daily.

During overnight raids, U.S. and Iraqi soldiers arrested four members of an unidentified Shiite “special groups cell,” who confessed to the bombing, U.S. spokesman Rear Adm. Gregory Smith told reporters.

“Based on subsequent confessions, forensics and other intelligence, the bombing was the work of an Iranian-backed special groups cell operating here in Baghdad,” Smith said, adding that he was not accusing Iran itself of ordering the blast.