U.S. and Iraqi forces captured more than 100 suspected guerrillas in raids across the country, arresting one of Saddam Hussein's (search) intelligence chiefs and another Iraqi believed involved in a homicide bombing last month, a U.S. commander said Thursday.

The raids on Wednesday and Thursday occurred as daily attacks on U.S. forces are climbing after a recent lull. Rebels lobbed a mortar shell Thursday at a checkpoint near Baghdad International Airport (search), killing one U.S. soldier and wounding another, the U.S. command said.

The attack outside the airport, which serves as a major American military base, brought to 529 the number of American troops killed since the Iraq war (search) began March 20.

American forces are also tracking a shadowy militant group that claimed responsibility for Sunday's back-to-back homicide bombings, said U.S. Army Brig. Gen. Mark Kimmitt. The bombings, which devastated gatherings at Kurdish political offices in the northern city of Irbil, killed at least 109 people, including senior Kurdish politicians who were strong U.S. allies.

A statement from a group calling itself the Ansar al-Sunna Army said it targeted the "dens of the devils" because of the Kurds' ties to the United States.

Kimmitt said the U.S. military believes Ansar al-Sunna is a splinter group of Ansar al-Islam (search), which is believed to be linked to the Al Qaeda (search) terrorist network.

The claim could not be confirmed, and the military said investigators from the military and the U.S. Federal Bureau of Investigation had made no progress in finding those responsible for the catastrophic bombings. But a Kurdish politician said its investigators were following clues picked up in the bombings' aftermath.

The Ansar al-Sunna statement may be significant for other reasons, however. Few previous attacks on U.S. forces and Iraqi civilians have been followed by claims by groups claiming to have arranged them. Most are chalked up to a little-understood guerrilla insurgency that bears no name.

"We are certainly going to follow up on the claims," said Kimmitt, the command's deputy chief of staff for operations. "That's the only claim we have right now. We aren't closer than we were 24 hours ago or 48 hours before ... either on the group responsible or on its motivation."

Kimmitt said Ansar al-Sunna -- which casts itself as protector of Iraq's Sunni Muslims -- also claimed responsibility for Saturday's car bombing outside a police station in the northern city of Mosul that killed nine people.

A U.S. official in Washington who spoke on condition of anonymity said Ansar al-Sunna is an umbrella organization for anti-U.S. extremists founded last September with Ansar al-Islam at its core. The group has claimed responsibility for several attacks in Iraq, many of which cannot be verified.

Among the assaults it claims are the Nov. 29 ambush that killed seven Spanish intelligence officers on a highway south of Baghdad and the Oct. 14 car bomb at the Turkish Embassy in Baghdad that killed two and wounded 13.

Kimmitt said Iraqi authorities detained one person suspected to have been involved in the Irbil bombing but quickly released him.

The military also counted a few high-level rebel suspects in a string of recent arrests.

Chief among them was former Brig. Gen. Abu Aymad al-Tikriti, who was head of military intelligence in northern Iraq under Saddam's regime, Kimmitt said. Al-Tikriti, believed to have commanded a guerrilla cell, was arrested along with three others near Saddam's hometown of Tikrit on Thursday.

On Wednesday, U.S. forces captured Majid Ali Abbas al-Dazi, suspected to have coordinated a homicide truck bombing Jan. 24 in the central town of Samarra, about 70 miles north of Baghdad.

That blast appeared to have targeted a government building. It killed four Iraqi civilians and wounded about 40 people, including seven American soldiers.

The number of daily attacks has climbed slightly, Kimmitt said. U.S.-led military forces faced an average of 24 daily attacks during the past week, while the week before, troops faced an average of 18 attacks daily.

Meanwhile, U.S. officials and a Shiite group close to Iraq's most powerful Shiite Muslim clergyman, Grand Ayatollah Ali al-Husseini al-Sistani, denied Arab media reports of an attempt on his life Thursday.

In the autonomous northern Kurdish region, two Kurdish parties ravaged by Sunday's homicide blasts in Irbil have begun their own investigation, said Fouad Masoum, of the Patriotic Union of Kurdistan.

"So far, investigators have been able to find some clues which they believe will lead them to the perpetrators or those who are behind them," Masoum told the AP, saying investigators believed one of the bombers who attacked the KDP office was a Yemeni.

He was identified by a taxi driver from a video taken by a cameraman for the reception. The driver told investigators he brought the Yemeni from Kirkuk to Irbil the morning of the blasts.

Masoum said Kurdish investigators questioned a hotel owner in Kirkuk who said the Yemeni man stayed in the Kirkuk hotel with another Yemeni, who left the same day. The hotel owners told investigators both were armed.