BAGHDAD (Reuters) - It was a brazen, complex attack worthy of Iraq's al Qaeda at its peak. Two bombs minutes apart killed, maimed and distracted while a team of suicide attackers blasted into a Baghdad police base to try to free jailed insurgents.
Tuesday's high-profile assault on a anti-terrorism police unit in Baghdad was the latest in a drive by the Islamic State of Iraq, al Qaeda local affiliate, to make good on a pledge to win back ground lost in its war with American troops - its leader has even threatened to strike at the United States.
Insurgents ultimately failed to free their prisoners, but the intended message was clear: we're back.
With Sunni Muslim militants trickling into neighboring Syria to battle President Bashar al-Assad, security experts say al Qaeda is reaping funds, recruits and better morale on both sides of the border, reinvigorating it after years of losses against U.S. forces and their Iraqi allies.
Islamic State of Iraq and other Sunni militant groups hate Assad's minority Alawite sect, a distant offshoot of Shi'ite Islam, which they see as a heretical oppressor of Sunnis.
Hostile to Shi'ites in general, they also oppose the Shi'ite-led government of Iraqi Prime Minister Nuri al-Maliki. Iran, the major Shi'ite power in the region, is a firm ally of Assad and wields great influence in Baghdad.
Al Qaeda appears to be exploiting Sunni-Shi'ite tensions fuelled by the increasingly sectarian conflict in Syria. Many Sunnis in Iraq are already disgruntled with what they see as Maliki's determination to minimize their share in power.
"The Syrian crisis is a venue in which an Iraqi-dominated al Qaeda branch is better able to attract fighters and resources to its cause," said Ramzi Mardini, an analyst with the Institute for the Study of War in Washington. "This may be a revival of confidence on the part of Sunni extremists."
Once at the heart of a Sunni insurgency against U.S.-led forces, al Qaeda lost many commanders to Iraqi and U.S. troops. Sunni tribes turned the tide against it from 2007, when they fought the group with U.S.-supplied guns, partly in revulsion at the indiscriminate carnage it had inflicted on civilians.
Iraq's violence has eased since the sectarian bloodbath of 2006-2007, but each month since the last American troops left in December al Qaeda has claimed responsibility for at least one major, well-coordinated attack.
A return to all-out sectarian violence looks unlikely, but the possible fall of Syria's Assad worries Iraqi Shi'ite leaders who fear that a hardline Sunni government could come to power instead, emboldening Sunni militants in Iraq.
Already, Baghdad says that seasoned al Qaeda fighters are crossing the 680 km (422 mile) border into Syria to liaise and conduct attacks on Assad's government. That hands Islamic State of Iraq new legitimacy in the eyes of some Sunnis, experts say.
Along Iraq's western frontier with Syria, near Albu Kamal, unrest across the border has fired up sympathies in a Sunni heartland with shared tribal and family ties.
Al Qaeda influence is strong in some remote border villages, and Iraqi forces skirmish daily with smugglers and insurgents sending fighters and weapons into Syria, said one senior Iraqi security official.
"The religious legitimacy of the Syria war and the increase of funding and fighters almost unquestionably benefits Al Qaeda in Iraq," said Seth Jones, a counter-terrorism expert at Washington's RAND Corporation and an author on al Qaeda. "It is heavily involved in overseeing the war in Syria."
America's withdrawal took with it U.S. intelligence gathering capabilities, handing insurgents more operational space in former strongholds such as Anbar province and areas where local political infighting hobbles Iraqi armed forces.
At the start of the Muslim holy month of Ramadan at the end of July, a rare communiqué from al Qaeda's local chieftain, Abu Bakr al-Baghdadi, announced a renewed jihad to recapture areas lost during the years of conflict with American forces.
In a fiery message about his muhahideen combatants,mujahedeenalso warned Americans: "You will soon see them in your homes. The war has only just started."
Even before his call to arms, June witnessed some of the severest attacks attributed to al Qaeda in Iraq since the U.S. pullout, including a surge in bombings on Shi'ite pilgrims and a suicide attack on a Shi'ite religious office in Baghdad.
July was Iraq's bloodiest month in the past two years with 325 people killed in attacks, most of them civilians.
Though al Qaeda's support had dwindled in Iraq because of civilian casualties, its bombings now focus on Shi'ite targets, government offices and local security forces as it seeks to inflame sectarian tensions and undermine Maliki.
The group, once a magnet for foreign fighters who often antagonized locals, has now reverted to a core of Iraqis hardened by the anti-American insurgency and in U.S. jails.
"These organizations have shown a resilience by adapting and forming small diffuse cells," said John Drake, at AKE Group consultancy. Al Qaeda in Iraq is now "seen as a much more domestic organization, so it could actually have more support among some members of the Iraqi public", he added.
Undeniably weaker and with less capability to hold territory than it did a few years ago, Iraqi officials acknowledge al Qaeda has crept back into old strongholds, aided by government inertia stemming from political infighting and corruption.
Mosul, a northern city in an area American soldiers once called the triangle of death, is considered the Islamic State of Iraq's unofficial capital, where officials say the group draws millions of dollars a month from extortion rackets.
In Baquba city, where political disputes among Sunni, Shi'ite and Kurdish leaders have paralyzed the provincial government, more than 20 neighborhood leaders known as mukhtars, who provide information to security forces, quit their posts in July for fear of al Qaeda.
"The reason for this wave of resignations is the armed operations by al Qaeda targeting mukhtars and their families," said Abdullah al-Hiyali, a local mayor.
Some Iraqi security officials play down al Qaeda influence and say evidence from several recent large-scale attacks suggests they were the work of other insurgents linked to political groups trying to destabilize the government.
They point to a wave of attacks that killed more than 100 people on July 23. Al Qaeda claimed responsibility, but none of the attacks involved the group's trademark suicide bombings. Instead multiple car bombs were detonated by remote control.
At least six Sunni insurgent groups, including ex-members of Saddam Hussein's former Baath party, have fought on after the U.S. withdrawal, whereas most Shi'ite militias disbanded or were incorporated into government security forces.
That makes for a complex mosaic of Sunni armed groups who are sometimes rivals and sometimes work together.
But Tuesday's attack in central Baghdad looked like classic al Qaeda. An initial car bomb drew in security forces, followed five minutes later by another blast, killing at least 25 people.
At least three gunmen wearing suicide belts and police uniforms, broke into the counter-terrorism headquarters after one of them detonated his explosives at the gate, according to the interior ministry, which blamed al Qaeda.
Only after a sustained gun battle inside the building did the security forces regain control.
"People were saying in 2009, this group is dead," said Daniel Byman, of the Saban Center at Brookings Institute who released a recent study on al Qaeda affiliates. "Even if they don't succeed in reclaiming territory, the fact they can plausibly contest that is the biggest sign of their success."
Additional reporting by Suadad al-Salhy and Raheem Salman in Baghdad; Editing by Alistair Lyon