MANSOURA, Egypt (AP) -- A powerful blast ripped through a police headquarters in an Egyptian Nile Delta city Tuesday while top security officials met to work out arrangements for an upcoming constitutional referendum, killing 15 people and wounding more than 100 in the deadliest bombing yet in a campaign of violence blamed on Islamic militants.
The attack underlined the vulnerability of Egypt's police and their weakness in keeping security amid fears of increased militant violence in the lead-up to the Jan. 14-15 referendum.
The vote is a key step in the country's political transition after the military's ouster of Islamist President Mohammed Morsi in July, but it has further stoked political tensions, with Morsi's Muslim Brotherhood protesting against the new charter.
Authorities quickly sought to pin blame for the blast on the Brotherhood, the military-backed interim government's top political nemesis. With the group continuing protests, the government has increasingly depicted it as directly behind the wave of violence, without providing evidence in public.
The attack hikes pressure on the government by anti-Islamists to take tougher action against the group, including enforcing a court-ordered ban on it, possibly declaring it a terrorist organization and passing a controversial harsh new anti-terrorism law. The Brotherhood condemned the attack and accused the government of scapegoating it.
At the funeral for the victims of the blast, including 14 policemen and a civilian, hundreds massed in a main square of the city of Mansoura where the bombing took place, chanting, "The people want to execute the Brotherhood."
They raised posters reading "no to terrorist groups" and pictures of military chief Gen. Abdel-Fattah el-Sissi, who removed Morsi and is the country's most powerful figure. Some in the crowd set fire to the car of a man who flashed a pro-Morsi symbol and others attacked houses of Brotherhood leading member.
Egypt has seen an escalating campaign of spectacular bombings, drive-by shootings, assassinations and mass killings, mainly against security forces, in the aftermath of Morsi's ouster and the subsequent deadly crackdown on the Brotherhood, which that left hundreds dead and thousands injured. Thousands of Morsi supporters have been arrested -- that latest, Hesham Kandil, the prime minister during Morsi's presidency, who was reported arrested Tuesday to serve a previously issued one-year prison sentence.
Most attacks have been centered in the Sinai Peninsula, where multiple militant groups operate, but the insurgency has been spreading to the capital Cairo and heavily populated Delta.
There was no immediate claim of responsibility for Tuesday's blast, but senior military and security officials said it carried the fingerprints of Ansar Beit al-Maqdis, or the Champions of Jerusalem, which has emerged as the main Sinai-based militant group. The group has claimed most of the biggest attacks the past months, including a failed attempt to assassinate the interior minister in Cairo in September. Only days earlier, the group threatened more attacks, saying it considers Egyptian troops and police to be infidels because they answer to the secular-leaning military-backed government.
The bombing at the security headquarters in Mansoura -- a provincial capital 110 kilometers (70 miles) north of Cairo that is considered a stronghold for the Brotherhood -- was the most significant yet outside the Sinai. The same building had been targeted in July, when an explosive planted outside killed a policeman and wounded another.
Tuesday's 1:10 a.m. blast brought down an entire section and side wall of the five-floor building. Dozens of parked cars and police vehicles were incinerated, and several nearby buildings were damaged, including a bank and theater. Windows of nearby houses were shattered.
Fifteen people were killed and 100 people were injured, Health Ministry spokesman Mohammed Fatahallah said. Two top officers were among the dead, and Mansoura's security chief was injured, losing an eye, the state news agency MENA reported.
Security officials were still trying to determine how the attack was carried out. The senior security and military officials said it may have involved a suicide bomber in an explosives-laden pick-up truck, possibly with more explosives planted outside the building. They spoke to The Associated Press on condition of anonymity to discuss the ongoing investigation.
They said that the attack shows "negligence" by police to protect themselves -- as well as the possibility that the attackers had inside information -- since it came as top provincial police officials were meeting to discuss security arrangements during the referendum. One security official said the military and police are struggling to track militants because of new recruits among Islamic extremists who have no previous records and files.
They also warned that as the military intensifies operations in Sinai, militants are moving attacks to other parts of the country, saying they have information about possible upcoming attacks targeting churches and vital institutions especially during the Coptic Christmas celebrations and holidays.
Appearing before a press conference, grim Prime Minister el-Beblawi stopped short of directly blaming the Brotherhood for the attack. But he grouped it in with the pro-Morsi protests as part of a string of "violations of the people's security."
He called the attack a "maximum offense" to Egypt and will be dealt with decisively, without elaborating. El-Beblawi said his government has been working to implement a court order in late September banning the Brotherhood. The presidency declared three days of nationwide mourning.
One of his spokesmen, Sherif Shawki, went further, accusing the Brotherhood, which he said showed its "ugly face as a terrorist organization, shedding blood and messing with Egypt's security," according to the state news agency MENA.
Authorities are considering declaring the Brotherhood a "terrorist organization," a move that would step up the legal moves against the group. 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 to avenge his ouster. Morsi supporters and rights groups have called the accusations implausible.
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."
Islamist thinker Kamal Habib, a former member of the militant Gamaa Islamiya group, said there is not yet evidence that militants like Ansar Beit al-Maqdis and the Brotherhood are working together, they share common goals."
"They want to undermine the police, spread fear and hinder the upcoming referendum," he said. "But this doesn't mean they are collaborating in the attacks."
He warned however that the more pressure the security authorities put on the group's youth, the more likely they cross the line of "extremism to violence."
"Now I see the group's youth with radical discourse. It's easy to cross the line to violence and join these terrorist groups," he said.
Michael reported from Cairo. Associated Press writers Maamoun Youssef and Sarah El Deeb contributed to this report.