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 suicide 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 suicide 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.