BAGHDAD — During the darkest days of the sectarian bloodletting that swept Iraq several years ago, few terrors equaled the rise of the death squads — men in the guise of the Iraqi security forces who killed with impunity.

The death squad killings not only inflamed sectarian fighting but also undermined the public’s confidence in its own security forces. Since then, tens of thousands of members of the security forces have been purged, and the Iraqi Army and police have struggled to regain public confidence as the American military presence recedes.

But in the past two weeks, there have been two attacks by men wearing Iraqi Army uniforms that have revived the specter of the death squads, stirring concern at the highest levels of the Iraqi and American commands.

Before dawn on Wednesday, men dressed in Iraqi Army uniforms stormed a house in a small village north of Baghdad, rounded up six members of a family, including women and children, and killed them, Iraqi officials said. The attack occurred nine days after men dressed in Iraqi Army uniforms raided a house near the Abu Ghraib district of Baghdad Province and killed 13 civilians.

Details of both episodes remain murky, with relatives, local officials and government security officials giving different accounts of the motivations of the attackers — and their theories often break along sectarian lines.

“We are witnessing the return of the death squads,” said Vice President Tariq al-Hashimi after the first attack.

Mr. Hashimi is engaged in a bitter struggle over a proposed election law that he has maintained disenfranchises Sunni voters, a fight that has largely occurred along sectarian lines and threatens to delay the first national elections in four years.

The political wrangling, which has left Sunnis feeling increasingly disenfranchised, is raising concerns that it could fuel a parallel rise in sectarian violence.

Both attacks occurred in areas that were once strongholds for Sunni insurgents, and all of the victims were Sunnis. There has been concern that militants are trying to regain a foothold in the areas around Baghdad to step up attacks before the elections, which were expected to be held in January.

One theory about the attacks is that militants are posing as members of the army to foment distrust among Sunnis, turning them against government troops, and in that way making it easier to establish havens. The government, however, has provided no evidence to this effect and the theory is based on little more than speculation by local security officials, who would speak only on the condition of anonymity.

The official statements from the Shiite-led government have been subdued, saying only that the killing of the 13 civilians resulted from “tribal disputes” and offering no immediate comment on Wednesday’s killings.

When asked about the earlier attack, Gen. Ray Odierno, the commander of American forces in Iraq, said that Americans were involved in the investigation and that several suspects had been detained.

“There are several different theories why that happened,” he said at a news conference last week. The suspects could be “some people who at one time were picked up for working with Al Qaeda and have since been released and went after the security forces that arrested them.”

“There is also a theory that it could be tribal,” he said. “That’s what we have to work our way through.”

Speaking about the attack on Wednesday, witnesses told local police officers that men in army uniforms stormed a house in Tarmiya, 20 miles north of Baghdad, shortly before dawn.

The assailants then killed six members of the family, according to a local police official, who said the victims’ throats were slit. An official at police headquarters in Baghdad confirmed the number of dead but said the victims had been shot.

“These people are terrorists and they wore the Iraqi Army uniforms to make the government look bad,” said a local police commander, who spoke anonymously because he was not authorized to speak with reporters. “This has nothing to do with sectarian issues.”

There were other attacks across Iraq on Wednesday, including two bombings in the Shiite holy city of Karbala that killed three people and wounded at least 36 others.