Dozens of military and police officers descended on the Iraqi town of Rutbah on Thursday, rounding up a number of men they said were responsible for the execution-style slayings of 22 Shiite pilgrims in the area earlier this week.

Army and police SWAT team members in tanks from the majority Shiite city of Karbala south of Baghdad rumbled through the largely Sunni area in Anbar province Thursday afternoon to take into custody the men they said were involved in Monday’s attack.

Gunmen on Monday night had stopped a tour bus of pilgrims from Karbala traveling from a holy shrine in Syria, hijacking the vehicle and driving it out into the desert. They killed all the men who had been on board.

It was unclear who authorized Thursday’s military action, and why. The number of men arrested was in dispute. A spokesman for the Karbala police said that 10 men were taken into custody, while authorities in Anbar said the number was 20.

The governor of Anbar province said Thursday that the round-up was a “kidnapping operation” done by “an act of gangs.” The governor, Qassem Mohammed Abed Hamadi, blamed Shiite officials in Karbala.

Police officials in Karbala, where the dead had been mourned in highly public funerals this week, drove through the streets in police cars with loudspeakers to spread what a police commander there called “the good news” of the arrests.

Ahmed Abu Risha, the Sunni leader of the Anbar Salvation Council, said that the force from Karbala that descended on his province Thursday modeled its methods on those of the kidnappers on Monday night.

“It’s a revenge sectarian action,” he said. Abu Risha has said he fears that such violence in the country will escalate as the U.S. military prepares to leave by Dec. 31.

Majeed is a special correspondent. Special correspondents Uthman al-Mokhtar in Fallujah and Saad Sarhan in Najaf contributed to this report.