BAGHDAD, Iraq – Two detainees may have been tortured to death at the hands of Iraqi security forces, the head of a commission investigating allegations of abuse at Iraqi jails said yesterday.

But the precise cause of their deaths is unclear, Iraq’s deputy prime minister, Rosh Shawais, who is heading the investigation, said in an interview.

Detainees told investigators that the two alleged victims were tortured or starved to death, while prison officials say the pair died of natural causes.

In all, American Ambassador Zalmay Khalilzad told reporters yesterday, at least 120 prisoners allegedly have been abused at the hands of Iraqi security forces, more than previously disclosed by the government of Iraqi Prime Minister al-Jaafari. As many as 18 prisoners may have died while in custody at a Baghdad detention center first identified last month in a Los Angeles Times report.

Mr. Shawais, an ethnic Kurd, said he is continuing to investigate reports of other deaths.

The new allegations have surfaced at the end of a heated and sometimes violent political campaign leading up to Thursday’s parliamentary vote. Yesterday, a Sunni Arab candidate for parliament, Mizhir Dulaimi, was killed and a prominent Shiite candidate was targeted in an assassination attempt.

Meanwhile, four American soldiers died yesterday after their patrol struck a roadside bomb northwest of the capital. Nearly 2,150 American service members have died in Iraq since the American-led invasion in March 2003.

Reports of prisoner abuse in Iraqirun jails first surfaced last month after American soldiers raided a compound in the Jadiriya section of Baghdad and discovered scores of men showing signs of hunger and torture.

Officials last week discovered another detention center where at least 13 prisoners showed signs of physical abuse. On Monday, a Sunni politician released a DVD purportedly showing tortured detainees at a third facility, though the allegation could not be confirmed.

Mr. Shawais has presented a report to the prime minister detailing preliminary findings regarding alleged torture at the first compound discovered last month. He said his committee will finish its final report before the end of the year. He requested an extension of the deadline in order to widen the scope of the investigation.

The allegations of abuses of suspected Sunni Arabs by security forces that are dominated by Shiite Muslims have become intertwined with the fierce political campaign, which came to an official halt yesterday to give voters a day to consider their choices.

A secular coalition led by a former interim prime minister, Ayad Allawi, which is running against the Shiite-led government, has alleged human rights conditions in Iraq are now as bad as under Saddam Hussein.

Mr. al-Jaafari, at a luncheon organized for journalists, vowed yesterday that his government would seriously examine the allegations. “We should provide prisoners with facilities that are up to international standards,” he said.

But he and his deputies also tried to minimize the issue, with one adviser arguing that Iraqis were far more concerned about bread-and-butter issues such as jobs, electricity, and water than the prisoner abuse scandal. Mr. al-Jaafari likened the reports of torture and starvation to allegations of prisoner abuse by American soldiers at Abu Ghraib, arguing, “It’s a matter of individuals, not the government.”

However, Mr. Khalilzad brushed aside attempts by some officials in the al-Jaafari government to downplay the torture allegations. “It was far worse than slapping around,” the American ambassador said, calling on the Iraqi government to speed up the investigation.

“We are very committed to looking at all the facilities,” he said. “It’s unacceptable for this kind of abuse to take place.”

The slain Sunni politician, Dulaimi, was gunned down yesterday morning as he was driving with a relative in the western Iraqi city of Ramadi. The relative was severely wounded.

Dulaimi was described as an active and dedicated political leader and Arab nationalist who took part in a recent conference on Iraq’s future held in Cairo, Egypt.

“Those who killed him are the enemies of Sunnis,” a relative of the victim, Aboud Hamadi, said.

An outspoken Shiite cleric running on Mr. al-Jaafari’s United Iraqi Alliance, Jalaluddin Saghir, also was the target of a failed assassination attempt on a road south of Baghdad, police said.

Several insurgent groups with ties to Al Qaeda have criticized the upcoming elections as a “satanic project.” But other insurgent groups more closely aligned with nationalist Sunni Arab tribal networks have urged Sunnis to vote. Sunnis form the backbone of the violent insurgency fighting the American military and the Shiite-led Iraqi government.