Updated

An explosion outside a Baghdad university as students were heading home for the day killed at least 65 people on Tuesday in the deadliest of several attacks on predominantly Shiite areas. The military announced the deaths of four U.S. soldiers.

Attacks in Baghdad — the university explosion, blasts at a marketplace for used motorcycles and a drive-by shooting — came as at least 109 people were killed or found dead nationwide in what appeared to be a final spasm of violence ahead of an imminent security operation by the Iraqi government and U.S. forces to secure the capital.

The violence also came a day after the Iraqi government hanged two of Saddam Hussein's henchmen in an execution that left many of the ousted leader's fellow Sunni Muslims seething after one of the accused, the ousted leader's half brother, was decapitated on the gallows.

The U.S. military also reported Tuesday that four American soldiers were killed in northern Iraq on Monday by a roadside bomb.

The blast struck the Task Force Lightning Soldiers assigned to the 4th Brigade, 1st Cavalry Division Monday while they were conducting operations in the northwestern Ninevah province, according to a statement. Ninevah is home to the city of Mosul, which has seen a recent increase in violence.

The identities of the slain soldiers were being withheld pending notification of their relatives.

Cabinet ministers and legislators loyal to the radical Shiite cleric Muqtada al-Sadr were instructed to end their six-week boycott of the political process, a parliamentarian in the political bloc said Tuesday, indicating that the decision was linked to the new security drive.

"We might be subjected to an attack and we should try solve the problem politically. We should not give a chance for a military strike against us," said the legislator, speaking on condition of anonymity because the information was not yet public.

The lawmaker said the group's return was conditional, including demands that the government set up a committee to establish a timetable for the withdrawal of U.S. troops and a second that would set a date by which Iraqi forces were to take control of security nationwide.

Until the walkout, the al-Sadr faction was an integral part of Prime Minister Nouri al-Maliki's governing coalition. Six Cabinet ministers and 30 legislators who belong to the movement called the boycott after al-Maliki met with President Bush in Jordan in late November.

Much of the violence has been blamed on Shiite militias, particularly the Mahdi Army militia loyal al-Sadr. Dozens of bodies turn up on the streets of Baghdad daily, many showing signs of torture.

Tuesday's largest attacks took place in primarily Shiite neighborhoods and appeared to be the work of Sunnis, who largely make up the insurgency targeting the Iraqi government and U.S. forces.

Raad Abbas, a 26-year-old who received shrapnel wounds in the attack at the motorcycle market that killed 13, said he went to the market because the city had been quieter over the past two weeks.

"Shortly after midday, I heard an explosion. Motorcycles were flying in the air, people were falling dead and wounded," he said from his hospital bed.

As the curious gathered to look at the aftermath of the first explosion — a bomb attached to a motorcycle — a suicide car bomber drove into the crowd and blew up his vehicle. The attack appeared to target the mainly Shiite neighborhood near the market but also was near the Sheik al-Gailani shrine, one of the holiest Sunni locations in the capital

The bombing near Al-Mustansiriya University took place as students were boarding minivans waiting outside the building to take them home, police said. Some police saying the explosion was caused by a suicide car bomber and others saying two of the minivans blew up as students were boarding.

About 45 minutes later, gunmen in a minivan and on two motorcycles opened fire on an outdoor market in a mainly Shiite neighborhood in nearby section of eastern Baghdad, police said. At least 11 people were killed.

While most of those killed were in Baghdad, two Christian brothers and a Sunni Arab mechanic were shot to death in two separate attacks in Mosul, police said. They also found the bullet-riddled body of a man in his 30s in the northern city, which also has seen a recent increase of violence.

In Monday's execution, a thickset Barzan Ibrahim plunged through the trap door and was beheaded by the jerk of the thick rope at the end of his fall, in the same execution chamber where Saddam was hanged a little over two weeks earlier.

Dozens of people, mostly schoolchildren, read Koranic verses at the graves near Tikrit, 80 miles north of Baghdad, as mourning continued for Ibrahim, Saddam's half brother and former intelligence chief, and Awad Hamed al-Bandar, head of Iraq's Revolutionary Court under Saddam.

Some 150 youths also staged a demonstration, chanting "down with the pro-Iranian government" and "glory to Barzan," and hundreds later assembled for a memorial service, but it was calmer than the day before when at least 3,000 angry Sunnis assembled for the burials in nearby Ouja.

A government video of the hanging, played at a briefing for reporters, showed Ibrahim's body passing the camera in a blur. The body came to rest on its chest while the severed head lay a few yards away, still wearing the black hood pulled on moments before by one of Ibrahim's five masked executioners.