BAGHDAD, Iraq (AP) – The woman slipped into the town, passing checkpoints where women are not searched. Then, donning a man’s “dishdasha” – a traditional white robe – and kaffiya headscarf, she blended in with the men waiting in line to join the Iraqi army.

She then set off the explosives strapped to her body, killing six would be recruits and wounding 35 – and sparking worries over a potentially dangerous new insurgent tactic.

The attack Wednesday in the northern town of Tal Afar, near the Syrian border 260 miles northwest of Baghdad, was the first successfully carried out by a female suicide bomber since Iraq’s bloody insurgency began.

The move exploits a hole in security that is tough to fill, especially ahead of an upcoming referendum on the country’s new constitution, in which men and women are expected to be lining up at the polls. Iraqi officials on Wednesday worried about having to step up searches of women at the numerous checkpoints that guard facilities across Iraq – a process that requires extra resources and irritates cultural sensitivities.

Gen. Ahmed Mohammed Khalaf, the regional police chief, said insurgents likely sent a woman from outside the area to carry out the attack because women are rarely searched at checkpoints entering Tal Afar “because of religious and social traditions.” Islamic customs frown on such close physical contact by men with women, so female personnel – who are few – must carry out any searches.

The Tal Afar attack also appeared aimed at showing that militants could still strike in a town where U.S. and Iraqi offensives drove out insurgents only two weeks before.

Iraq’s most notorious insurgent group, al-Qaida in Iraq, claimed responsibility for the attack in an Internet statement, saying it was carried out by a “blessed sister.”

The bombing came a day after U.S. and Iraqi officials announced their forces killed the second-in-command of al-Qaida in Iraq, Abdullah Abu Azzam, in a raid in Baghdad over the weekend. The killing has not slowed insurgent violence, with at least 84 people – including seven U.S. service members – killed in attacks since Sunday.

President Bush warned violence will increase in the days leading up to the Oct. 15 referendum. The draft constitution that the U.S. hopes will be approved has sharply divided Iraq’s Shiite Muslim majority and Sunni minority, who form the backbone of the insurgency.

“We can expect they’ll do everything in their power to try to stop the march of freedom,” Bush said. “And our troops are ready for it.”

The U.S. military announced Wednesday that two more American soldiers and an airman were killed in violence and a Marine was killed by a non-combat gunshot. The deaths brought to 1,922 the number of U.S. service members who have died since the Iraq war started in March 2003, according to an Associated Press count.

In the holy city of Najaf, south of Baghdad, an attacker set off an explosion in the home of a bodyguard of radical Shiite cleric Moqtada al-Sadr on Wednesday, killing two people and wounding five, al-Sadr aides and a hospital official said.

In the attack at the Tal Afar army recruitment center, the female suicide bomber disguised herself in a traditional white “dishdasha” robe and a checkered kaffiya headscarf – both worn only by men – to stand in the line of Iraqi applicants, Maj. Jamil Mohammed Saleh said.

She detonated explosives packed with metal balls and hidden under her clothes, Saleh said.

In a photo of the attacker’s head taken by Saleh and shown to AP, the woman appeared to be in her early 20s with dark eyes, light skin and brownish hair. Saleh said it was not known whether she was Iraqi.

U.S. and Iraqi troops swept through Tal Afar in a Sept. 8-12 offensive, with Iraqi authorities claiming nearly 200 suspected militants were killed and 315 captured, though many of the insurgents in the town escaped. Since then the bulk of the forces participating in the offensive withdrew, though a U.S. base remains.

Though it was the first known successful attack, it was not the first attempt by a woman to carry out a suicide bombing.

In March, four women, reportedly sent by the insurgent group Islamic Army in Iraq, were caught in a town south of the capital before they could set off explosives belts they were wearing. Before the insurgency began, Saddam Hussein’s regime is known to have used female suicide bombers at least once: Just before the April 2003 fall of Baghdad, two women detonated their car near the city of Haditha, killing three American soldiers.

Women and children will now be searched at Tal Afar checkpoints, Khalaf, the police chief, told AP.

Still, the attack raised the prospect of more women bombers being used by the insurgency, a tactic difficult to defend against, especially during the referendum. Men and women turned out in large numbers to vote in parts of Iraq during January parliamentary elections, and images of veiled women flashing their ink-stained fingers after voting became an iconic symbol of hopes for democracy.

Maj. Gen. Hussein Ali Kamal, intelligence head at the Iraqi Interior Ministry, said the Tal Afar attack “rings danger alarms” and requires new techniques, including increased searches of women at sensitive locations.

“But this will be a problem, because women are taking part in our new political life and finding large numbers of female security officers to search them is not an easy process,” he told AP.

In the past, women have played only a supportive role in the insurgency, helping smuggle equipment or feed, shelter and give medical treatment to fighters, said Nora Bensahel, an insurgency expert with Rand Corp., a nonprofit research group based in Santa Monica, Calif.

“This could be a sign that the insurgency is getting greater support among a larger segment of the population, that women are getting more militant and willing to take on a greater role,” Bensahel said. “It could also be a sign that the insurgents are having trouble finding male recruits.”

AP writers Omar Sinan in Baghdad and Jim Krane in Dubai, United Arab Emirates, contributed to this report.