BAGHDAD, Iraq — Insurgents exploded a suicide car bomb and launched two mortar shells at Iraq's Interior Ministry during National Police Day celebrations today, killing 21 people and injuring 24, police said.

An Internet statement by Abu Musab al-Zarqawi in the name of his al-Qaida in Iraq terrorist group rebuked Sunni Arabs for taking part in last month's parliamentary elections, saying they had "thrown a rope" to save U.S. policy.

Meanwhile, the U.S. military said eight U.S. troops and four American civilians died aboard a U.S. Army Black Hawk helicopter that crashed late Saturday in northern Iraq. The military initially said only that there were eight passengers and four crew aboard.

With the latest military deaths, at least 2,207 U.S. service members have died since the war started in 2003, according to an Associated Press count.

The military said it wasn't yet known what caused the crash and an investigation would take some time. The helicopter went down about seven miles east of Tal Afar, a northern city near the Syrian border that has seen heavy fighting with insurgents.

The Black Hawk was part of a two-helicopter team providing support for the 101st Airborne Division and was flying between bases when communications were lost, the military said.

It was the deadliest helicopter crash in Iraq since a CH-53 Sea Stallion went down in bad weather in western Iraq on Jan. 26, 2005, killing 31 U.S. service members.

The attack on the Interior Ministry began with a suicide car bomber who exploded his vehicle near an entrance checkpoint. Less than an hour later, two mortar rounds landed about a half-mile from where police were gathered to mark National Police Day.

At least 21 people were killed and 24 injured, mostly policemen, said police Sgt. Abdel Hadi Hassan. Several police cars were destroyed in the explosions, and pieces of body parts could be seen on the ground.

In other violence today, gunmen assassinated an investigative judge in Kirkuk, police Capt. Farhad Talabani said. In Baghdad, gunmen fired on three people working on Iraq's de-Baathification commission, killing one, police Capt. Qassim Hussein said. Gunmen also killed an Iraqi intelligence officer and a doctor in separate attacks, Hussein said.