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BAGHDAD, Iraq (CNN) -- Two explosions rocked western Baghdad's al-Shu'lah neighborhood near a Shiite Muslim mosque on Sunday, killing at least 15 people and wounding at least 57, Iraqi police said.

The first blast was triggered near the mosque -- and when people gathered near the scene, a suicide car bomber drove into the crowd and detonated the vehicle in the second blast, police said.

Word of the blasts in the capital came just hours after news of two suicide car bombs in Tikrit, about 90 miles (150 km) north of Baghdad.

The bombs exploded just 15 minutes and a short distance apart, killing at least six people and wounding 26 at an Iraqi Police Academy in Tikrit, according to an official with Tikrit's governor's office.

Police were responding to the first explosion -- which happened in front of the police academy at 8 a.m. (12 a.m. EDT) -- when the second car bomb detonated close by at the meteorology building, the official said.

In an earlier attack Saturday evening, seven commandos with Iraq's Interior Ministry were wounded when five mortar rounds landed inside their facility in the al-Baiya' neighborhood of southwest Baghdad, Iraq police said.

The attack in Tikrit, Saddam Hussein's hometown, occurred as new recruits at the academy were about to travel to the Jordanian capital of Amman for a training program, police Lt. Shalan Allawi said, The Associated Press reported.

A doctor at Tikrit General Hospital said the bombs killed four policemen and two civilians, with 23 policemen and several civilians wounded, AP reported.

Elsewhere, three insurgents were killed Sunday as the roadside bomb they were trying to plant in the town of Mahawil exploded, police said in the nearby city of Hillah.

The explosions follow the deaths of at least 12 people Saturday in a series of attacks by insurgents.

The U.S. military said Task Force Baghdad soldiers arrested eight people Saturday. They are suspected of shooting down a commercial helicopter Thursday.

The military said in a release that an "Iraqi civilian helped Task Force Baghdad soldiers find" eight people, who were being questioned in the crash that resulted in the deaths of 11 people on board.

Six American security contractors, two Bulgarian crew members and two Fijian security guards were killed in the crash.

A Bulgarian crew member who survived the crash was shot to death, according to the Bulgarian company that owned the helicopter.

The helicopter was flying from Baghdad to the northern city of Tikrit when it went down just north of the capital.

A news release from Task Force Baghdad said an "Iraqi citizen told the soldiers he knew where the blue Kia pickup truck the terrorists used during the attack was parked and led them to the site.

"When the soldiers got there, several other local residents confirmed the first tip and showed the soldiers where the terrorists lived."

Soldiers searched two houses at 12:30 a.m. The military said troops captured three men and bomb-making materials at one house, and three more men suspected of making bombs at a second home -- but it did not specify where the houses were.

The Americans who were killed in the crash worked for North Carolina-based Blackwater Security Consulting USA. Friday, the company released the names of the employees who died.

They are: Robert Jason Gore, 23, of Nevada, Iowa; Luke Adam Petrik, 24, of Conneaut, Ohio; Jason Obert, 29, of Fountain, Colorado; Steve McGovern, 24, of Lexington, Kentucky; Eric Smith, 31, of Waukesha, Wisconsin; and David Patterson, 27, of Havelock, North Carolina.

Violence intensifies

Insurgents kept up attacks on security forces Saturday and Sunday.

The weekend attacks are the latest in an apparent surge of rebel violence following a slight decline in February and March. As the attacks continue, Iraqi officials are struggling to put together a new government chosen in the January 30 elections for a 275-member transitional national assembly.

This week, more than 50 people have been killed and more than 100 others wounded in the most notorious attacks. Insurgents primarily have targeted military and police officers, but have also gone after government officials and contractors.

CNN military analyst and retired Brig. Gen. David Grange said Thursday that he expects more spikes in violence.

"I think the recent violence is typical," he said. "And there will be continual spikes throughout the next several years as this transition to Iraqi security forces and this Iraqi governance takes place.

"A lot of Iraqi civilians are reporting the whereabouts and the networking of these insurgent forces.