Story Highlights• U.S. probing possible Iranian involvement in brazen compound raid
• Speculation centers on military branch known for taking revenge
• Iraq at "precarious juncture," State Department nominee says
• Bombs, gunfire and mortar attacks target Shiite pilgrims
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BAGHDAD, Iraq (CNN) -- The Pentagon is investigating whether a recent attack on a military compound in Karbala was carried out by Iranians or Iranian-trained operatives, two officials from separate U.S. government agencies said.
"People are looking at it seriously," one of the officials said.
That official added the Iranian connection was a leading theory in the investigation into the January 20 attack that killed five soldiers.
The second official said: "We believe it's possible the executors of the attack were Iranian or Iranian-trained."
Five U.S. soldiers were abducted and killed in the sophisticated attack by men wearing U.S.-style uniforms, according to U.S. military reports. (Watch how attackers got into the compound )
Both officials stressed the Iranian-involvement theory is a preliminary view, and there is no final conclusion. They agreed this possibility is being looked at because of the sophistication of the attack and the level of coordination.
"This was beyond what we have seen militias or foreign fighters do," the second official said.
The investigation has led some officials to conclude the attack was an "inside job" -- that people inside the compound helped the attackers enter unstopped.
Investigators are looking particularly at how the attackers got U.S.-style military uniforms and SUVs similar to those used by U.S. troops. (Watch what could happen if the U.S. opts to strike Iran )
"'Who was behind it all?' was the fundamental question," the first official said.
The U.S. military on Friday confirmed accounts that the soldiers had been abducted and driven away from their compound. The military had said in a January 20 press release only that "five U.S. soldiers were killed and three wounded while repelling the attack."
Some Iraqis speculate that the Iranian Revolutionary Guard Corps carried out the attack in retaliation for the capture by U.S. forces of five of its members in Irbil, Iraq, on January 11, according to a Time.com article published Tuesday. (Read the article)
The Iranian Revolutionary Guard Corps has a reputation for taking harsh and unrelenting revenge on its enemies, the Time.com article says.
The five Iranians are still in U.S. custody.
The U.S.-led coalition has said a preliminary investigation found links between the detainees and the Iranian Revolutionary Guard, which has provided funds, weapons, bomb technology and training to extremist groups in Iraq.
The Bush administration has authorized U.S. forces to kill or capture Iranian agents plotting attacks in Iraq, a U.S. national security official said Friday.
"If Iran escalates its military actions in Iraq to the detriment of our troops and/or innocent Iraqi people, we will respond firmly," Bush said Monday in an interview with National Public Radio.
During a hearing Tuesday, some U.S. senators expressed concern over the Bush administration's position against directly engaging Iran and Syria diplomatically in an effort to reduce violence in Iraq, contrary to recommendations issued in December by the Iraq Study Group.
"What I think many of us are concerned about is that we stumble into active hostilities with Iran without having aggressively pursued diplomatic approaches," Democratic Sen. Barack Obama said. (Full story)
Violence targets Shiite pilgrims
Meanwhile Tuesday, a string of attacks on Shiite pilgrims in Iraq and a mortar strike on a Sunni neighborhood in Baghdad killed at least 48 people and wounded more than 100.
The violence comes on the final day of Ashura, when Shiite Muslims mark the seventh-century martyrdom of the Imam Hussein, the Prophet Mohammed's grandson. (Watch why rising Shiite power raises concerns )
People filled the streets of Karbala, the Shiite holy city south of Baghdad, to commemorate Ashura.
The violence was farther north in Baghdad and Diyala province.
Two major attacks occurred Tuesday in Diyala, a religiously and ethnically mixed province northeast of Baghdad.
In Balad Ruz, about 50 miles northeast of Baghdad, a suicide bomber blew himself up at the main gate of the Ali al-Akbar mosque, killing at least 19 people and wounding 54, said police in Baquba, the provincial capital.
In Khanaqin, about 100 miles northeast of Baghdad, a roadside bomb exploded among a group of Shiite pilgrims. At least 11 were killed and 33 wounded, police said.
As many as 1.5 million Shiite pilgrims who couldn't get to Karbala for the holy day trekked instead to Baghdad's Kadhimiya district, site of the Musa al-Kadhim shrine.
In southwestern Baghdad's Amil neighborhood, two cars filled with gunmen opened fire around midday on a minibus of pilgrims believed headed to Kadhimiya. Seven people were killed and seven wounded, according to the Interior Ministry.
Also, nine people were wounded in a mortar attack in the district, police said.
Earlier, a mortar round hit a Shiite mosque in northern Baghdad, killing one person and wounding two.
The U.S. military late Tuesday said U.S. aircraft shot at an insurgent mortar team firing in the direction of the Musa al-Kadhim shrine at midafternoon. It was not known whether there were casualties.
Later in the day, 10 people were killed and 16 wounded when 10 mortar rounds were fired at the Adhamiya district, a Sunni neighborhood in northern Baghdad, police said.
The bodies of eight people thought to be victims of sectarian violence were found throughout Baghdad on Tuesday.
CNN's Terence Burke, Jomana Karadsheh, Barbara Starr, Mohammed Tawfeeq, Ted Barrett and Nicky Robertson contributed to this report.
An Iraqi soldier guards the government compound in Karbala where disguised attackers struck January 20.