BAGHDAD, Iraq (CNN) -- Attacks in Iraq and the crash of a Black Hawk helicopter brought to 25 the number of American troops killed in Iraq on Saturday, making it one of the deadliest days for the U.S. military since the war began almost four years ago.

The U.S. military is investigating the cause of the helicopter crash northeast of Baghdad that killed 12 soldiers.

However, a senior military officer said the initial indications point to hostile fire as the cause of the crash.

The highest single-day U.S. military death toll was recorded on January 26, 2005, when 37 American troops died, comprising 31 from a Marine helicopter crash that was ruled accidental and six combat deaths.

Five U.S. soldiers died in Karbala during an attack on a meeting to ensure the security of pilgrims flocking to the city for the Islamic holy month of Muharram.

An armed militia group using grenades, small arms and indirect fire attacked the meeting, the U.S. military said Sunday. Three soldiers were wounded "while repelling the attack," the military said in a statement.

Karbala governor Akil al-Khazali told The Associated Press the attackers drove sport utility vehicles and wore both Iraqi and "foreign" military uniforms -- though he did not specify which foreign uniforms.

During Muharram, pilgrims take part in the rites of Ashura commemorating the martyrdom of Hussein, the grandson of the Prophet Mohammed.

The 25 deaths on Saturday -- and two others on Friday -- bring the total for U.S. military deaths in January to 51 and the number since the Iraq war began to 3,055, including seven civilian contractors of the Defense Department.

In addition to the helicopter crash, troops were killed Saturday by roadside bombs, wounds sustained in combat, and a grenade attack, the military said.

Friday's deaths were caused by a roadside bomb and "enemy action."

New U.S. deployment to Iraq under way

The U.S. military announced Sunday that about 3,200 U.S. soldiers are being deployed to Baghdad to assist Iraqi security forces in the coming weeks -- marking the first part of President Bush's plan to add 21,500 troops to the war effort.

The 2nd Brigade, 82nd Airborne Division, which had been on standby in Kuwait, has been added to Multi-National Corps-Iraq and will be fully operational "on or about" February 1, the U.S. military said in a news release.

Its mission will be to help Iraqi forces secure Baghdad, where the security situation has deteriorated in recent months.

"Soldiers from the 82nd come to us ready to engage in a wide variety of operations, in support of the Iraqi Baghdad security plan," said Lt. Gen. Ray Odierno, commander of Multi-National Corps-Iraq.

Shiite cleric al-Sadr's political bloc ends boycott

Ending a two-month boycott, the powerful political movement of anti-American Shiite cleric Muqtada al-Sadr will return to Iraq's parliament, the parliamentary speaker announced Sunday.

Politicians backing al-Sadr withdrew participation in Iraqi politics in a protest over Prime Minister Nuri al-Maliki's November meeting with President Bush in Jordan.

The al-Sadr bloc controls six government ministries and holds 30 of the 235 seats in parliament.

At a joint news conference with members of the al-Sadr bloc, parliamentary speaker Mahmoud al-Mashhadani said the boycott ended after negotiations with parliament members who said they would consider the group's demands -- which al-Mashhadani called "national demands."

Those demands include setting a timetable for withdrawal of U.S.-led coalition troops from Iraq.

Al-Mashhadani said the Iraqi parliament will assign three of its committees to work on a plan to present to parliament members.

U.S. commanders blame the Mehdi Army, a militia loyal to al-Sadr, for much of the sectarian violence that has wracked Baghdad and other cities over the past year.

The Iraqi government has found it difficult to rein in al-Sadr's fighters, since his political backing helped put al-Maliki into office.

But a newly implemented Baghdad security plan spearheaded by al-Maliki has aimed at cracking down on both Shiite militias and Sunni insurgents.

"Nobody will be untouchable here in Iraq now," Iraq's government spokesman Ali al-Dabbagh told CNN Friday. "Everybody will be subject to the law, and whether he is from any party or any sect, whether he's Sunni or Shiite, the plan will not exclude anyone."