A car packed with explosives detonated outside a security headquarters building in this city north of Cairo early Tuesday, killing 15 people and wounding more than 130 in one of the deadliest militant attacks in Egypt in years.

The powerful blast, which occurred just after 1 a.m. in a cramped quarter of Mansoura, was probably the work of a suicide bomber, Egypt’s Interior Ministry said in a statement late Tuesday.

There was no immediate assertion of responsibility for the bombing, which killed 11 police officers and four civilians, according to state media. But the attack demonstrated the growing sophistication of Egypt’s insurgency nearly six months after the ouster of Islamist President Mohamed Morsi in a military coup.

The explosion brought down an entire section of the five-story building that housed local police forces and other security agencies, leaving many employees buried under the rubble. It also tore the concrete walls off a neighboring bank, spilling its contents into the street.

More than 12 hours after the attack, shards of glass from shattered storefronts crunched under the feet of visibly shocked residents and shop owners sifting through the debris.

“I came here last night and everything was on the ground,” said 28-year-old Ashraf Fateh Sayed, the owner of a cellphone shop just yards from the security building. “It was chaos — there was looting. We are alive, but what can we do now?”

Residents of this Nile Delta city about 70 miles north of Cairo took to the streets to call for revenge against the Muslim Brotherhood, the Islamist organization that backed Morsi and that many in Mansoura blame for the violence.

In a statement shortly after the attack, a spokesman for Prime Minister Hazem el-
Beblawi said the group had showed its “ugly face as a terrorist organization.”

Egypt’s interim military-backed government has repeatedly accused the Muslim Brotherhood of “terrorism” but has provided no evidence that the Islamist group, which renounced violence decades ago and whose leaders are in prison, is responsible for any of the recent attacks on security forces in Egypt.

In Washington, State Department spokeswoman Jen Psaki said the United States “condemns in the strongest possible terms today’s terrorist attack on the Dakaliya security directorate in Mansoura.” She said the United States “stands firmly with the Egyptian people as they work to put their country on the path towards democracy, stability and economic prosperity, in an atmosphere free from violence.”

Ahmed Ali, a spokesman for the Egyptian army, posted on his Facebook page Tuesday that the attack in Mansoura has made the military more determined to “purge the land” of extremists.

The Muslim Brotherhood, in a statement released by the group’s office in London, also swiftly condemned Tuesday’s attack. It accused Egypt’s government of trying to “exploit” the crime.

In Mansoura, known for its anti-Islamist sentiment, crowds gathered near the bombing site and chanted loudly against the Brotherhood, blasted pro-military songs and later set fire to cars and what they said were Brotherhood-linked businesses.

“We want the execution of the Muslim Brotherhood,” Shadia Hagras, a resident, said. “We want to eliminate them.”

But analysts tracking Egypt’s rising insurgency said the Sinai-based jihadist group Ansar Bayt al-Maqdis, also known as Ansar Jerusalem, is more likely to be behind such a bold and complex attack.

The group, although established in the Sinai, has asserted responsibility for a number of deadly car bombings in the peninsula and in the Egyptian heartland since August, when security forces violently dispersed two pro-Morsi sit-ins in Cairo, killing about 1,000 people. Among the attacks was a car bombing that targeted a police compound in the Suez Canal city of Ismailia on Dec. 12.

In a statement released Monday, Ansar Bayt al-Maqdis said it considers members of the Egyptian military and police to be “infidels” because they are loyal to the current, secular government.

After the Mansoura bombing, pharmacist Wael Abdel Rahman Fawzi, 26, whose brother’s clinic was destroyed in the explosion, said, “I feel like we are in Iraq, not in Egypt.” The engine from the car used in the attack tore through the window and landed on the clinic’s first floor.

“The people pay the price,” Fawzi said.

Sharaf al-Hourani in Cairo and Abigail Hauslohner in Beirut contributed to this report.