U.S. troops arrested four Iraqi men in the kidnapping of American journalist Jill Carroll who was held captive for 82 days, a U.S. spokesman said Wednesday, as troops reinforcements were moving into the capital to stem sectarian violence that threatens civil war.

Maj. Gen. William Caldwell said the four, who were not identified, were arrested in Anbar province west of Baghdad but he did not say when. Another U.S. spokesman, Lt. Col. Barry Johnson, said the arrests were made about a month ago.

Caldwell also said a U.S. Army helicopter crashed in Iraq's western Anbar province Tuesday, leaving two crew members missing and four injured. It did not appear the helicopter was shot down, the military said.

The UH60 Blackhawk helicopter with six people aboard went down in water and divers were searching for the missing soldiers, Caldwell said. He did not identify the body of water.

In Baqouba, 35 miles northeast of Baghdad, four people were killed and 16 wounded in an explosion late Tuesday in a residential area that collapsed three buildings and damaged a mosque, police said.

Johnson, the U.S. spokesman, said the blast originated either inside the collapsed buildings or nearby. He said one person was killed and 17 injured. The discrepancy in the toll could not be immediately reconciled.

Caldwell said Carroll, who works for the Christian Science Monitor, was held at three locations, including one about 7 miles west of Fallujah before she was freed March 30.

The 28-year-old journalist was kidnapped Jan. 7 in west Baghdad and her Iraqi interpreter was shot dead. She was released near a Sunni Arab political party office in Baghdad and returned to the United States on April 2.

The breakthrough came after a Marine lieutenant identified a house from intelligence reports and photographs he had seen. The lieutenant and others in his unit questioned the owner of the house, which led to identification of additional locations where Carroll was believed to have been held, Caldwell said without elaborating.

"Sound intelligence was paramount here," he said.

Caldwell said other Marines went to a second location and arrested one person. Three others were arrested at yet another place north of Abu Ghraib in a raid in which two hostages were freed.

Four of the detainees were arrested for involvement in Carroll's kidnapping. The role of the fifth suspect was unclear. A decision on their prosecution will be taken shortly, he said.

Carroll's father, Jim Carroll, said in a statement "we are pleased to hear about the arrests." The Christian Science Monitor also said the organization is "reassured" to hear of the arrests.

The kidnappers, a formerly unknown group calling themselves the Revenge Brigade, had demanded the release of all female detainees in Iraq, saying Carroll would be killed otherwise. U.S. officials did release some female detainees but said the decision was unrelated to the demands.

On Wednesday, two people were killed and the bodies of three men were found. In New Zealand, the Foreign Ministry said a Cook Islands national working as a driver in Iraq was killed in a bomb attack late Tuesday.

Deputy Health Minister Dr. Sabah al-Husseini said about 1,500 violent deaths were reported in the Baghdad area last month, which showed a steady increase in killings since the beginning of the year.

Much of the violence has been blamed on sectarian militias that have stepped up a campaign of tit-for-tat killings since the Feb. 22 bombing of a Shiite shrine in the northern city of Samarra.

The ongoing violence in Baghdad has prompted U.S. commanders to reinforce troop strength in the city.

A U.S. statement Tuesday said about 6,000 additional Iraqi troops were being sent to the Baghdad area, along with 3,500 soldiers of 172nd Stryker Brigade Combat Team and 2,000 troops from the U.S. 1st Armored Division, which has served as a reserve force since November.

Some of the reinforcements already have been seen patrolling a mostly Sunni neighborhood in western Baghdad, scene of armed confrontations between Sunni and Shiite gunmen.

Caldwell described the troops deployment as a gradual and continuous process, adding that for the security plan to succeed it needs the cooperation of Iraqis.

"They have to be involved. Military force alone cannot achieve peace. It can only set the conditions to allow for peace to take hold and grow," he said.

Many of the militias responsible for sectarian violence are linked to political parties that are part of Prime Minister Nouri al-Maliki's national unity government, and they are reluctant to disband their armed wings unless others do the same.

The U.S. ambassador to Iraq, Zalmay Khalilzad, said talks were under way between various Sunni and Shiite groups to reach agreements and sign pledges to end sectarian fighting.

Also Wednesday, Romanian President Traian Basescu arrived in Baghdad to meet Iraqi and U.S. officials and visit some of the country's 890 troops stationed there. Basescu was received by President Jalal Talabani and will meet other key U.S. and Iraqi officials.