BAGHDAD (AP) – Two aides to Grand Ayatollah Ali al-Sistani were killed in shootings within hours, prompting his Basra followers to boycott Friday sermons in protest amid fears that an internal Shiite power struggle was increasingly targeting Iraq’s top Shiite cleric.

A top aide to Prime Minister Nouri al-Maliki, meanwhile, conceded it may prove difficult for the Iraqi government to expel Western security contractors despite outrage that followed the killings of civilians in a shooting involving Blackwater USA contractors protecting State Department personnel.

The aide, who spoke on condition of anonymity because the investigation into Sunday’s incident was ongoing, said a way out of the Blackwater crisis could be the payment of compensation to victims’ families and an agreement from all sides on a new set of ground rules for their operations in Iraq.

An Interior Ministry spokesman, Maj. Gen. Abdul-Karim Khalaf, said Friday that a report had concluded that Blackwater guards opened fire from four positions on a square in western Baghdad after a vehicle near their convoy failed to stop.

Iraqi witnesses and officials have offered several conflicting versions of events and it was not clear how the Interior Ministry report would affect a joint U.S.-Iraqi investigation.

Al-Sistani’s followers in Basra, 550 kilometers (340 miles) southeast of Baghdad, refused to attend Friday sermons in their mosques, denouncing the latest assassinations of the cleric’s associates, an aide said.

Al-Sistani’s representative in the Diwaniyah province, Ahmed al-Barqaawi, was gunned down while driving home to the city of Diwaniyah, about 130 kilometers (80 miles) south of Baghdad, police officials said.

Hours earlier, one of the cleric’s representatives in the Basra area, Amjad al-Janabi, was killed along with his driver in a shooting west of the southern city, police said.

The deaths bring to at least five the number of al-Sistani aides slain since early August but it remains unclear if the killings reflect internal Shiite disputes or are the work of Sunni insurgents opposed to the vast influence enjoyed by al-Sistani over Iraq’s Shiites and politics since Saddam Hussein’s 2003 ouster.

Al-Sistani’s office in the holy city of Najaf declined to comment on the latest slayings. Basra Gov. Mohammed al-Waili called on the government to step up measures to protect clerics.

Rival Shiite groups clashed violently in August in another Shiite holy city, Karbala, during a religious festival that left at least 52 people dead.

Tensions have also increased in Baghdad, where the shooting incident Sunday involving Blackwater USA security guards which Iraqi officials said left at least 11 people dead in Nisoor Square in western Baghdad, infuriated many Iraqis.

American and Iraqi officials announced a joint committee to probe the widely differing versions of the incident. Khalaf said the Interior Ministry report found that the security guards opened fire first on Iraqis who were driving in their cars.

The report, Khalaf said, recommended annulling a legal provision that gives immunity to foreign security companies operating in Iraq. It also recommended Blackwater pay compensations to the victims’ families and that all foreign security companies be replaced by Iraqi security companies.

According to Khalaf, a car bomb detonated around noon Sunday near al-Rahman mosque in Mansour, a mile (half a kilometer) north of Nisoor Square. “Minutes later, two mortar rounds landed nearby Nisoor square and they (Blackwater) thought that they were under attack,” Khalaf said.

“They started shooting randomly from four positions in the square, killing 11 civilians and injuring 12 others. The first one who was killed was a driver who failed to stop and then his wife,” Khalaf said, adding his opinion about the foreign security guards: “They always lose their cool and have their fingers on the trigger.”

Separately, authorities in the autonomous Kurdish region in northern Iraq called for the release of an Iranian detained by U.S. forces Thursday in Sulaimaniyah.

The U.S. military said he was smuggling in roadside bombs as a member of the elite Iranian paramilitary Quds Force, which is accused by the United States of arming and training Shiite militias in Iraq. But a statement by the Kurdish government said the Iranian was part of an Iranian delegation of economists and businessmen, with an “official invitation.” A spokesman, Fuad Hussein, said the detention was “illegitimate.”

The U.S. detentions of Iranians is a sensitive subject for Iraqi officials trying to balance the interests of their rival U.S. backers and Iran, powerful allies of the Shiite-led government.

Kurdish authorities also were irked by the January arrest of five Iranians during a U.S. raid in the northern city of Irbil.

U.S. authorities have said the five included the operations chief and other members of the Quds force. Iran has insisted the five were diplomats in Iraq with permission of the government.

The arrest could further strain Washington-Tehran relations, already taxed by earlier detention of each other’s citizens, as well as U.S. accusations over Iranian involvement in Iraq’s violence and Iran’s disputed nuclear program.

Iran has denied allegations that it is stoking the violence.