For Iraqis, bombings and shootings like those that killed more than 100 people throughout the country on Monday have become a grim part of daily life after the departure of U.S. troops last December.

The question facing U.S. officials is whether the mass killings, which have accelerated throughout the summer, represent a return to sectarian war or a resurgence of al-Qaida.

The attacks, spread across 13 cities and more than 40 locations, targeted mostly Shiite neighborhoods and appeared to be the work of al-Qaida in Iraq, a militant Sunni group. The carnage included an assault on a military base with guns and grenades, a car bombing in a Shiite vegetable market and a suicide bombing by an assailant who detonated his explosives in a crowd of police officers rushing to help Iraqis injured in earlier bombings.

Although Iraq typically sees a spike in violence during the Islamic holy month of Ramadan, which began last week, Monday's attacks were among the most coordinated that the country has seen in the past several years.

"The size and frequency of these attacks tells me that al-Qaida is returning and re-establishing networks in Iraq," said retired Lt. Gen. James Dubik, who oversaw the training of Iraqi forces in 2008. "Things in Iraq are definitely not fine."

More than 570 Iraqis have been killed in major attacks this year, a significant uptick in violence in the wake of the U.S. departure from the country.

Over the weekend, the leader of Iraq's al-Qaida affiliate warned that the network was returning to its old strongholds and put out an open call for new recruits to launch attacks against the Shiite-led government and its security forces.

"The majority of Sunnis in Iraq support al-Qaida and are waiting for its return," said Abu Bakr al-Baghdadi, head of the Islamic State of Iraq since 2010. His statement was posted on a militant website.

The most recent attacks come amid an ongoing political crisis that began when Nouri al-Maliki, Iraq's Shiite prime minister, accused the country's Sunni vice president of taking part in a recent terrorist plot against the government. The unrest in neighboring Syria, which is increasingly seen as a battleground in a larger sectarian conflict, has increased tension inside Iraq, analysts said.

"What's happening in Syria is having a contagion effect," said Emma Sky, who served as a senior civilian adviser to the U.S. military in Iraq.

Although the revolt in Syria began as an uprising against Bashar Assad's regime, it has taken on an increasingly sectarian hue in recent weeks, heightening sectarian tensions throughout the region.

"Disaffected Sunnis see this terrible slaughter and feel a sympathy that al-Qaida is able to exploit," Sky said. "It doesn't mean that everyone will take part in the violence, but it does mean that people can bribe their way through checkpoints a little more easily."

Thus far the Iraqi army and police forces have not launched reprisal attacks into Sunni neighborhoods following the al-Qaida strikes as they did when sectarian violence in Iraq was at its worst.