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

The question facing U.S. officials is whether the mass killings, which have accelerated this summer, represent a return to sectarian war or a resurgence of Iraq’s al-Qaeda affiliate.

The attacks, spread across 13 cities and more than 40 locations, targeted mostly Shiite neighborhoods and appeared to be the work of al-Qaeda 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 blasts.

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 the country has seen in the past several years.

“The size and frequency of these attacks tells me that al-Qaeda is returning and reestablishing 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-Qaeda affiliate warned that the network was returning to its old strongholds and called for new recruits to launch attacks against the Shiite-led government and its security forces.

“The majority of Sunnis in Iraq support al-Qaeda 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 Web site.

Although U.S. intelligence officials concede that al-Qaeda has been able to exploit the departure of U.S. troops, they said they see no evidence that the group has been able to win over disaffected Sunnis in key strongholds such as Iraq’s Anbar province, which was the heart of the Sunni insurgency from 2003 to 2008.

“AQI remains isolated and, right now, its attempts to stir up sectarian violence aren’t having the desired effect,” said a U.S. official, who spoke on the condition of anonymity to discuss sensitive intelligence.

A White House spokesman stressed that Iraq’s army and police forces “have the capacity to handle their own security.”

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 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 al-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-Qaeda 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 after the al-Qaeda strikes, as they did when sectarian violence in Iraq was at its worst. Shiite militias have not reemerged in Iraqi cities while Shiites have relied on the Iraqi government to protect them.

The danger is that Iraqi society will put up with the violence for only so long before fissures open.

“What this shows is that the Iraqis don’t have the top-tier counterterrorism forces that the U.S. military has,” said Douglas Ollivant, a lead planner for the U.S. military in Baghdad.

The United States could use private contractors or CIA personnel to provide some counterterrorism training. But U.S. officials have limited leverage with Maliki, who is more focused on consolidating power than compromising with Sunnis and Kurds.

“At the margins, we can try to encourage calm and encourage Maliki to be less aggressive,” said Stephen Biddle, a senior fellow at the Council on Foreign Relations. “But to an important degree at this point, we are spectators.”

Greg Miller in Washington, Jabbar Yaseen in Baghdad and Babak Dehghanpisheh in Lebanon contributed to this report.