Uniformed attackers driving military trucks and armed with a car bomb, guns, grenades and suicide belts blasted their way into a provincial government headquarters in the northern city of Tikrit on Tuesday, killing at least 53 people in a highly organized raid, according to witnesses and local officials.

Over several hours, the attackers went room to room, tossing grenades down hallways and through doorways and killing local politicians and government workers with shots to the head, according to Iraqi security forces and two witnesses who escaped by jumping out of a second-floor window. More than 90 people were wounded, officials said.

After an hours-long firefight, Iraqi security forces — who called in U.S. helicopters and soldiers for support — entered the building, but there were no survivors inside. The attackers had either been shot or blown themselves up.

Sabah al-Bazee, 30, a freelance Iraqi journalist who worked for the Reuters news service and other media, was among those killed, Reuters reported.

Iraqi officials immediately blamed the attack on al-Qaeda-linked insurgents bent on destabilizing Iraq’s fragile government. Tikrit, a predominantly Sunni area and the home town of Saddam Hussein, was long a stronghold of groups with ties to al-Qaeda, although they have been relatively quiet in recent years.

For many Iraqis, the scale and apparent coordination of the attack brought to mind the siege of a Catholic church in Baghdad in the fall — also blamed on al-Qaeda-linked insurgents — that killed 68 people.

According to a doctor at Tikrit’s Salahuddin Hospital, 53 people were killed Tuesday, while an official at a morgue put the death toll at 75.

Although small explosions and, increasingly, assassinations occur almost daily across Iraq, security has improved dramatically. In general, Tikrit and surrounding Salahuddin province have become one of the country’s the most secure areas. When attacks come, however, they are often on a large scale. In January, a suicide bomber killed at least 50 people gathered at a police recruiting center in Tikrit.

Still, Tuesday’s carnage horrified Iraqis, many of whom fear that violence will worsen after U.S. forces withdraw as scheduled in December.

“What is this?” yelled Maj. Ali Ghalib, an Iraqi soldier whose cousin, also a soldier, was killed in the attack. “How did they not provide security? How did this happen?”

The raid began early Tuesday afternoon when the assailants — witnesses cited 10 or more — easily passed a first checkpoint into the provincial government compound and drove straight toward the headquarters of the provincial council, which meets on Tuesdays.

As they were questioned by security guards in front of the building, the attackers shot the guards in the head and then charged into the building, spraying gunfire on workers and civilians running out, according to local officials.

When an Iraqi security forces team arrived at the outside gate of the compound, a car parked there exploded, killing at least a dozen people. Among them were civilian relatives of people inside the building, who had called them for help.

As the attackers moved into the building, shooting and throwing grenades, people jumped out of second- and third-floor windows, breaking arms and legs as they landed. Seven workers barricaded themselves inside an office by shoving furniture in front of the door.

“People who tried to run away were shot,” said Hassoun al-Jubouri, 55, an engineer who was among the group. “We heard explosions going off and gunshots.”

As the building caught fire, Jubouri and his colleagues used a water cooler to douse the door, then managed to break a window and shout for help. Civilians outside leaned a ladder against the building, allowing him and six others to escape.

Outside, about 1,000 members of Iraq’s security forces had surrounded the building and called in U.S. forces for support, according to Iraqi and U.S. officials. U.S. surveillance helicopters hovered overhead, and American soldiers — it was not clear how many — were on the ground.

When security forces finally shot their way into the building, they found bodies of government employees, including many women, lying in hallways and offices with gunshot wounds to the head.

Several of the attackers had been shot, and others were charred to death, apparently having detonated belts with bombs.

Saif is a special correspondent. Special correspondent Ali Qeis in Baghdad contributed to this report.