(CNN) -- The United States denounced the early morning twin bombings at an Egyptian security forces building that killed 16 people and wounded more than 100 in the Nile Delta city of Mansoura.
In its statement on Tuesday, the U.S. State Department expressed hopes for a quick recovery for those injured in the two blasts, which occurred in quick succession about 1 a.m. local time Tuesday. State news agency MENA confirmed the casualty toll.
"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," the State Department message said.
Interim Prime Minister Hazem al Beblawi called the attack a terrorist incident and said the perpetrators will not go unpunished.
Ansar Jerusalem, a jihadist group responsible for attacks in Egypt's Sinai Peninsula, claimed responsibility for the act.
But al Beblawi singled out the Muslim Brotherhood, without directly blaming the organization for the attack, and called it a terrorist group in a statement to the nation, the state news agency MENA reported.
Among the people injured is the head of the security directorate of Dakahlia governorate, MENA said, citing Health Ministry officials. Part of the building collapsed after the explosion, the news agency said.
One blast occurred on one of the top floors of the building and was quickly followed by a car bomb, MENA reported.
Brotherhood labeled a terror group
Egypt's Cabinet declared the Muslim Brotherhood a terrorist group and a terrorist organization, Hossam Issa, deputy prime minister and minister of higher education, announced in a statement on Egypt's state TV on Wednesday.
Issa blamed the Brotherhood for the bombing, saying that "all of Egypt was horrified by the ugly crime."
Egypt's Cabinet also decided to place penalties on those who joined the group or remained a member.
A 'strong' condemnation from Brotherhood
The Muslim Brotherhood and its Freedom and Justice Party "strongly condemned" the bombing, the group said on its website.
It expressed surprise at al Beblawi's contentions.
"Coup authorities realized that fraud will not work (in the coming referendum on a new constitution). So, they resorted to this heinous bombing," Hamza Zawbaa, FJP media spokesman, said.
"All violence is reprehensible. God, as well as the whole world, bear witness that the Brotherhood is innocent of all that."
Witness: Heavy damage at the site
Ahmed El Shabrawy was at his home when he heard a loud blast. He told CNN it took him five minutes to run to the building, where he saw a man pulled from the rubble by bystanders. He estimated that dozens of cars were damaged, and there was a large area around one of the blast sites where there was "heavy material damage."
At least 10 ambulances had responded to the scene, he said. So many people rushed to a blood donation center that workers had to turn people away, he added.
CNN first learned of the blasts from Twitter.
Upcoming vote on proposed constitution
The blasts come in the lead-up to Egypt's referendum on a new constitution, which will be held January 14-15.
The draft constitution would ban religious parties and put more power in the hands of the military.
A military coup in July unseated President Mohamed Morsy, who was democratically elected.
Since then there have been almost daily protests, some of which have ended in violence and other bombings, such as one on December 12 when 20 police recruits were injured in a car bomb attack.
On Sunday, Ansar Jerusalem issued an online statement calling on army and police members to quit.
"With your staying in these institutions from evening to morning, you are incurring the anger of Allah," the statement said.
Morsy, an Islamist leader, has endured a series of legal troubles. He and 132 others will face trial for escaping from prison in 2011, state media reported Saturday.
Morsy will be tried along with members of the Muslim Brotherhood and Palestinian Hamas, the al Ahram state newspaper reported.
He is also charged with raiding other prisons, and killing soldiers and officers in Rafah, it said.
He has been in custody since his ouster.
CNN's Saad Abedine contributed to this report.