BAGHDAD (Reuters) - Masked gunmen stopped the bus full of Shi'ite Muslim police officers and families at what looked like an Iraqi army checkpoint on a western desert highway in the Sunni-dominated province of Anbar.
Two of the gunmen asked passengers one by one where they were from and then shot 14 dead in their seats, leaving one woman alive with a simple message: "Go back and tell them how we are killing you."
Last week's bus attack, whose details were recounted by local officials, strengthened fears that Iraq is edging back into sectarian mayhem, with al Qaeda again striking at will in a drive to provoke civil war.
Baghdad has now banned off-duty officials, police and soldiers from using the desert highway without an escort.
More than 70 people were killed on Monday alone when car bombs and attacks hit cities across northern Iraq. One assault on a police base involving suicide bombers, rockets and gunmen killed 40 people, mostly police and soldiers.
Invigorated by Syria's Sunni-led revolt and fed by Sunni frustrations at home, al Qaeda's Iraqi wing and other insurgents pose a violent challenge to Baghdad's Shi'ite-led government.
Iraqi officials say that in the desert near Syria men with black jihadi flags are reclaiming their former strongholds to use as staging posts in their deadly campaign.
Suicide bombers wearing explosive belts - a signature of al Qaeda - are hitting with a frequency not seen in years, implying there is no shortage of recruits ready to sacrifice themselves.
Nearly 2,000 people have died in attacks since April, according to U.N. figures, in the worst spike of bloodshed since Shi'ite-Sunni bloodletting eased five years ago.
Al Qaeda's local wing, Islamic State of Iraq, may spearhead the violence, but other Sunni armed groups are also resurgent, including the Naqshbandi army, an expanding network of Saddam Hussein's outlawed Baath party members and ex-army officers.
A few years ago Sunni tribal leaders turned on al Qaeda, disgusted by its indiscriminate killings and cadre of foreign fighters, helping U.S. troops defeat the insurgency in Anbar.
But with the Americans gone, Iraqi forces no longer have the air cover and intelligence that U.S. forces once supplied.
Iraqi political tensions have also favored an al Qaeda revival, with Sunni resentment against perceived discrimination by the Shi'ite-led government fuelling insurgent recruitment.
Shi'ite Prime Minister Nuri al-Maliki now accuses some Sunni tribal sheikhs of sheltering al Qaeda militants.
"There are those who want to incite sedition by targeting a certain sect," he said in an interview this week. "Yes, al Qaeda has a presence, but the painful thing is some tribes do not cooperate. These gangs are protected by some tribes."
Al Qaeda's revival in Iraq worries Washington, which pulled out its troops at the end of 2011. Shi'ite leaders in Baghdad also fear that the conflict in Syria may bring to power Sunni Islamists who might encourage Sunni rebels in Iraq.
Iraqi government officials say Shi'ite militias, which once engaged in tit-for-tat killings with Sunni insurgents, have yet to be drawn into a new cycle of sectarian revenge.
Sunni mosques, neighborhoods and tribal leaders have also been attacked - in some cases perhaps by al Qaeda provocateurs. But some Sunni leaders blame Shi'ite militias.
Al Qaeda and other insurgents no longer control vast areas or towns as in the heyday of their campaign against U.S. troops, and Sunni support for armed rebellion is far from universal.
Yet anger among minority Sunnis over perceived slights and abuses has deepened, leading to months of mass protests. After a deadly army raid on a protest camp in the town of Hawija in April, Sunni gunmen appeared in Falluja, Mosul and other cities and fought street battles with government forces.
One Sunni lawmaker, who asked not to be named, said militants were steadily gaining strength. "They are recruiting now among the protesters, and the government has no response."
Al Qaeda fighters use abandoned hamlets in Anbar's Jazira desert for safe haven and temporary shelter, often with the compliance of local communities, security officials say.
"Tribes won't revolt against them as long as they do not target their people," said one senior police officer who asked not to be named. "They say protecting soldiers and policemen is not their responsibility, it's the government's."
Apart from al Qaeda, which is partly focused on helping fellow-Sunni militants in Syria, other groups are trying to channel Sunni discontent into armed revolt against Baghdad.
In January, Izzat Ibrahim al-Douri, head of the Baath party and of the Army of the Men of the Naqshbandi Order, formed in 2007 to fight U.S. troops, urged Sunnis to rise against Maliki.
The Naqshbandi army plays on Sunni grievances over a law used by Iraq's Shi'ite leadership to exclude Baathists and officers of Saddam's disbanded army from public office. Limited concessions by Maliki have done little to defuse the anger.
Naqshbandi is now extending its influence beyond its traditional base in the northern cities of Kirkuk and Mosul, often in coordination with al Qaeda, security officials say.
Despite - or perhaps because of - its associations with deposed strongman Saddam, Naqshbandi has some advantages over the Islamist militants linked to global jihad, according to Ramzy Mardini, at the Iraq Institute for Strategic Studies.
"(It) is an indigenous movement and better integrated in Iraq's Sunni landscape than al Qaeda in Iraq," he said.
(Editing by Alistair Lyon)