BAGHDAD, Iraq — A suicide bomber triggered a ball bearing-packed charge yesterday, killing at least 41 people at a mostly Shiite college whose main gate was left littered with blood-soaked student notebooks and papers amid the bodies.

Witnesses said a woman carried out the attack at the business school annex to Mustansiriyah University, but Interior Ministry officials said it was investigating the reports. The school’s main campus was hit by a string of bombings last month that killed 70 people.

The attack came as a powerful Shiite militia leader bitterly complained that “car bombs continue to explode” — despite an ongoing security crackdown in Baghdad — and suggested that he was rethinking his cooperation.

The statement issued in the name of the radical cleric Moqtada al-Sadr put increased strains on the American-Iraqi security sweeps — aimed at restoring order in the capital. The cleric said any crackdown that includes American soldiers was doomed to failure.

Mr. Sadr’s Mahdi army militia pulled its fighters off the streets under intense government pressure to let the 12-day-old security plan proceed. But a relentless wave of Sunni attacks — six alone in the Baghdad area yesterday — has apparently tested Mr. Sadr’s patience as well as many ordinary Shiites.

A return to the streets by the Mahdi army forces could effectively block the security effort and raise the chances of Baghdad falling into sectarian street battles — the apparent aim of Sunni extremists seeking any way to destroy the American-backed government.

“Here we are, watching car bombs continue to explode to harvest thousands of innocent lives from our beloved people in the middle of a security plan controlled by an occupier,” a statement read by a Sadr aide in Baghdad said.

Mr. Sadr — who has not appeared in public in more than a month — is no friend of Washington, and his forces fought fierce battles with American troops in 2004. But he has largely cooperated in the Iraqi political process to avoid strains with Prime Minister al-Maliki and the Shiite leadership.

The statement, however, was highly critical of the American role in the security plan and urged leaders to “make your own Iraqi [security] plans.” He said, “No security plan will work” with direct American involvement.

The statement — read to hundreds of cheering supporters — was the first public word from Mr. Sadr since America’s assertions earlier this month that he fled to neighboring Iran to avoid arrest. Mr. Sadr’s aides and other loyalists insist that he never left Iraq.

Shiite anger at America is running high since American soldiers on Friday detained the son of the most powerful Shiite political leader for nearly 12 hours after he crossed from Iran. American officials say Shiite groups, including the Mahdi army, receive weapons and aid from Iran. Iran denies the charges.

“To my Shiite and Sunni brothers, I say, ‘Let us scorn sectarianism and hoist the banner of unity,'” said the statement from Mr. Sadr, whose militia is blamed for frequent execution-style slayings of Sunni rivals.

Since the security crackdown began, the number of bodies thought to be victims of Shiite death squads has gone down dramatically in Baghdad, but there has been no respite from violence blamed on Sunni insurgents.

Besides the college blast, at least 18 people were killed — mostly in Shiite districts — in bombings and rocket attacks in the Baghdad area.

Security guards at the Mustansiriyah University annex scuffled with the bomber before the blast, witnesses said. Most of the victims were students, including at least 46 injured, said police

If the bomber is found to be woman it would unusual — but not unprecedented — in Iraq’s chaos. The main campus at Mustansiriyah, about 1 1/2 miles away, was the target of twin car bombs and a suicide blast last month that killed 70 people.

The students at the business college were returning to midterm exams after the Iraqi weekend.

A 22-year-old student, Muhanad Nasir, said he saw a commotion at the gate. “Then there was an explosion. I did not feel anything for 15 minutes, and when I returned to consciousness, I found myself in the hospital,” Mr. Nasir, who was wounded in the head and chest, said.