A second homicide bombing (searchin as many days killed up to 47 people Wednesday, pushing the toll in the back-to-back attacks to 100. Again, Iraqis were the targets — this time, a crowd of volunteers for Iraq's new army — in an apparent campaign to wreck U.S. plans to transfer power by summer.

The U.S. military posted a $10 million bounty on a Jordanian militant suspected of organizing violence by foreign fighters and plotting an acceleration in attacks aimed at sparking a Sunni-Shiite civil war in Iraq.

The United States made public a letter to Al Qaeda leaders thought to be sent by the militant, Abu Musab al-Zarqawi (search). In it, he warns that militants are in a "race against time" to stop the June 30 handover of power, when Iraqi security forces will take a stronger role in battling the insurgency. The military announced Monday that it intercepted the document.

On Thursday, the U.S. command said a roadside bomb killed two American soldiers in Baghdad. The blast went off Wednesday evening in a western neighborhood of the capital, killing the soldiers from the 1st Armored Division (search), a spokesman said.

The deaths bring to 374 the number of Americans who have been killed in hostile action since the beginning of military operations in Iraq. A total of at least 534 Americans have died, including non-combat deaths.

In Wednesday's attack in Baghdad, an Oldsmobile packed with 300 to 500 pounds of explosives drove up to a crowd of Iraqis waiting outside an army recruitment center — only a few blocks from the heavily fortified Green Zone, headquarters of the U.S. administration.

The driver detonated the explosives, killing 47 people and wounding 55, the U.S.-led coalition said. The Iraqi Interior Ministry put the toll at 46 dead.

The aim Wednesday was clearly to kill Iraqis working with the U.S.-led coalition, rather than a particular religious group, because the crowd was likely a mix of Sunnis and Shiites.

But the homicide bombing Tuesday targeted a mostly Shiite town, Iskandariyah, south of the capital. A truck carrying a similar amount of explosives blew up outside a police station, killing 53 Iraqis, including would-be recruits lined up to apply for jobs.

There was no claim of responsibility for the rare consecutive attacks, but Maj. Gen. Charles H. Swannack Jr., commander of the 82nd Airborne Division, said he saw a connection between al-Zarqawi — and his memo — and the recent bombings.

"Iskandariyah is right on the line between Sunni and Shiite, so the attack there might be trying to foment some kind of civil war," said Swannack, whose division is based in the town.

A U.S. official in Washington said al-Zarqawi's involvement could not be ruled out, but that the blasts were more likely the work of supporters of Saddam Hussein. "They view police in training to be collaborators with the U.S.," the official said, speaking on condition of anonymity.

A prominent Sunni Muslim cleric, Hareth al-Darri, called al-Zarqawi "an imaginary character" and expressed doubt that he was playing a central role in the insurgency.

"Our position on resistance in Iraq is that of any Muslim whose land is occupied," al-Darri, head of the Muslims' Scholars Committee of Iraq, said in an interview Wednesday with Al-Jazeera television. "Our (Islamic) law commands us to resist the enemy. ... The resistance is mostly Iraqi. It is nationalist, that is, seeking to liberate its land which is legitimate."

Mohsen Abdel-Hamid, president of the U.S.-appointed Governing Council, said the attackers "want to undermine security so that independence will be delayed." The frequency of attacks may also be a "message" to a U.N. team of experts now in Iraq to assess whether an early election can be held before the transfer of power, he said.

Wednesday's was at least the ninth major vehicle bombing in Iraq this year — and U.S. officials say that as the June 30 deadline nears, more attacks will likely follow.

The Americans have portrayed the letter from al-Zarqawi as a sign of insurgents' desperation to stop the handover. The letter complained that Iraqi guerrillas have not cooperated enough with foreign Islamic fighters and said attacks would be tougher to carry out once Iraqi security forces take a stronger role.

"The noose is beginning to tighten around the necks of the mujahedeen, and the future is frightening with the future deployment of more troops and police," it says. If the insurgency fails to prevent the handover, "then there will be no choice but to pack our bags and move to another land."

It describes Iraqi soldiers and police as an instrument of the Americans, and "God willing, we are determined to target them forcefully in the coming period ... "

The letter also outlines a strategy of kidnappings of U.S. soldiers and greater attacks on "collaborators," Kurds and particularly Shiites, saying "the best solution" is to spark war between Iraq's Shiite majority and Sunni minority.

Insurgents have mounted a string of car and homicide bombings in recent weeks — the deadliest in the northern city of Irbil on Feb. 1, when two bombers blew themselves up at Kurdish party offices, killing at least 109 people.

Since Jan. 1, at least 261 Iraqi civilians have been killed in major homicide attacks or car bombings, according to an AP tally based on reports issued by the U.S. military or Iraqi police. Neither the Iraqi interim government nor the U.S. military provides comprehensive figures on Iraqi casualties nationwide.

At least 532 U.S. service members have died since the beginning of military operations in Iraq, according to the Department of Defense. Of those, 372 died as a result of hostile action.

The Baghdad recruitment center was surrounded by barbed wire with sandbagged posts in front. But around 300 Iraqis were gathered outside the center's locked gates, and were completely exposed when the car exploded at about 7:25 a.m. Some were lined up to join the military, others waiting to depart for training in Jordan.

"I was just telling my buddy that it was very dangerous to be standing here," said Ali Hussein, 22. He lay on a bed soaked in his blood at Karkh Hospital, his body shaking as he gasped for air. "Then I felt nothing but fire."

Another of the wounded, Abbas Hussein, 39, an army veteran looking to re-enlist, said the would-be volunteers in line "were all happy and excited."

"I wanted to rejoin because I love my country, the great Iraq," he said. "I wanted to protect the people."