MANSOURA, Egypt — A powerful blast tore through a police headquarters in an Egyptian Nile Delta city early Tuesday, killing 13 people, wounding more than 100 and leaving victims buried under rubble in the deadliest bombing yet in a months-long wave of violence blamed on Islamic militants.
Investigators were trying to determine whether the blast, soon after midnight in the city of Mansoura, was from a car bomb or from explosives planted around the building. The explosion left a downtown street of the city strewn with piles of debris and charred cars.
Egypt has seen an escalating campaign of spectacular bombings and gun attacks, mainly against security forces, since the military ousted Islamist President Mohammed Morsi in July and launched a fierce crackdown on his Muslim Brotherhood. Most have been centered in the Sinai Peninsula, where multiple militant groups operate, but the insurgency has been spreading to the heavily populated Delta and the capital, Cairo.
The interim government quickly blamed "dark terrorist forces" for Tuesday's attack. A government spokesman went further and accused Morsi's Brotherhood of orchestrating the bombing and called it a "terrorist organization."
Authorities appeared to be moving closer to officially declaring the Brotherhood a terrorist group. A court has already banned the group, but a terrorism designation would further escalate the crackdown against what was once the country's strongest political organization, winning elections the past three years and dominating the government during Morsi's one year presidency.
A government committee was meeting later Tuesday to review the group's status. Social Solidarity Minister Ahmed el-Borai, who is among those in charge of the review, said declaring it a terrorist organization was inevitable, saying the Brotherhood has "no consideration for the blood of innocents."
In a statement Tuesday, the Brotherhood condemned the bombing as a "direct attack on the unity of the Egyptian people." It accused the government of "exploiting" the violence to target the group and "create further violence, chaos and instability."
Since the coup, carried out after massive nationwide protests demanding Morsi's removal, Egypt's military-backed interim government has sought to portray the Brotherhood as largely responsible for the violence and militant attacks that engulfed the country following the 2011 ouster of longtime autocrat Hosni Mubarak.
Last week, prosecutors referred Morsi and other top Brotherhood leaders to trial on charges of organizing a large terrorist conspiracy, working with Hamas, Hezbollah, Iran and other militant groups and orchestrating the Sinai insurgency in revenge for his ouster.
The Brotherhood and its Islamist allies have been holding near daily protests demanding Morsi's reinstatement, which often descend into clashes with security and anti-Brotherhood civilians. The protests have been met by a crushing crackdown that has killed hundreds of protesters and jailed thousands. At the same time, the army and security forces have been waging an offensive in Sinai against militant groups. Officials say more than 180 suspected militants and more than 170 policemen have been killed in violence the past months.
The attack on the police station in Mansoura, seen as a stronghold of Brotherhood support 110 kilometers (70 miles) north of Cairo, was the first major bombing in the Nile Delta.
The 1:10 a.m. blast struck at the security headquarters, collapsing an entire section and side wall of the five-floor building. Dozens of parked cars were incinerated, and several nearby buildings were damaged, including a bank and theater.
Associated Press video from the scene showed bulldozers clearing the rubble outside the security headquarters, as charred and wrecked cars littered the street.
The dead included nine policemen, including two officers, and four civilians, and 101 people were wounded, Health Ministry spokesman Mohammed Fatahallah said. Among the injured were the city's security chief — who lost an eye — and his assistant, the state news agency MENA reported. Most of the victims were policemen, many of whom were buried beneath the debris.
Egypt's Interior Minister Mohammed Ibrahim toured the scene of the explosion at daybreak, pledging that the police will "continue their battle against the dark terrorist forces that tried to tamper with the country's security," then went to hospital to visit the wounded.
"This is a regrettable, horrible attack and whoever carried it out is not human," Ibrahim told reporters.
Prime Minister Hazem el-Beblawi described the attack as a "terrorist incident," expressed condolences to the families of the victims and vowed that the perpetrators "will not escape justice."
His spokesman Sherif Shawki pointed the blame at the Brotherhood. MENA quoted him as saying the group showed its "ugly face as a terrorist organization, shedding blood and messing with Egypt's security."
An unidentified senior security official told MENA that a pick-up truck laden with a large amount of explosives is suspected to be behind the attack. He said investigators are still looking to see whether it was detonated by timer or remote control.
But another official, speaking to The Associated Press on condition of anonymity because he was not authorized to speak to reporters, said the bomb could have been planted outside the building, saying no traces of a bomb vehicle or the explosive device itself had been found yet.
The same building had been targeted in July, when an explosive planted outside killed a policeman and wounded another.
Security forces cordoned off the area around the bombing site, closed major entrances and exits to Mansoura and set up checkpoints to search for perpetrators. State TV called on residents to rush to hospitals to donate blood.
No one immediately claimed responsibility for the bombing.
A day earlier, an al-Qaida-inspired group called on police and army personnel to desert. The group, Ansar Beit al-Maqdis, or the Champions of Jerusalem, threatened more attacks on the military and police, saying it considers Egyptian troops to be infidels because they answer to the secular-leaning military-backed government.
The group has carried out suicide bombings and other attacks in Sinai, but gained particular notoriety by striking outside the peninsula in recent months. It claimed responsibility for a failed attempt to assassinate the interior minister with a suicide bombing against his convoy in Cairo in September. The minister escaped unharmed.
Ansar Beit al-Maqdis is believed to have ties with Palestinian militants in the neighboring Gaza Strip, and officials have said other foreign militants have found refuge in Sinai during the ongoing turmoil.
Egypt's turmoil comes as the country nears a Jan. 14-15 referendum on a revised constitution, a key step in a military-backed transition plan leading to presidential and parliamentarian elections later next year. Morsi's supporters bitterly oppose the new document, which amends the constitution passed under his rule. But the interim government is pushing for an overwhelming passage of the draft to show the legitimacy of the military's ouster of Morsi and the post-Morsi political system.
Shawki, the Cabinet spokesman, vowed that "such terrorist operations will not prevent us from moving forward with the road map."
Michael reported from Cairo, and Associated Press writers Maamoun Youssef and Sarah El Deeb contributed to this report.