Articles I've been reading today indicated they were mostly Sunnis who moved to Karbala to begin attacks against Shiites during their religious holiday.
http://news.bbc.co.uk/1/hi/world/middle_east/6307587.stm
Intense clashes have been taking place in orchards outside the holy city of Najaf, as Iraqi troops backed up by US forces battled several hundred armed men, whose identity is as yet unclear.
The provincial governor of Najaf said a US military helicopter had been shot down by what he called "enemy fire" in Zarqa, scene of the clashes, and Iraqi police were quoted as saying the two crew on board were dead.
A Reuters correspondent reported seeing an American helicopter trailing smoke after a burst of machine gun fire.
Thousands of pilgrims descend on Karbala for Ashura
US military officials would not comment on what they termed an ongoing operation but said the reports were being checked.
Concerns over security have been heightened as Shia Muslims mark the religious festival of Ashura, which in previous years has been hit by co-ordinated attacks.
Some 10,000 Iraqi police and security forces have stepped up patrols in Karbala, where Shia pilgrims have been converging.
Security has also been tightened along the 110km (70 miles) of highway from Baghdad to Karbala that runs through the Sunni areas, the French news agency AFP reports.
Ashura draws huge numbers of Shias to Karbala from across Iraq and beyond its borders.
I'm confused. Do Muslims attack each other more or less during their religious holidays?