Photos are pictured on the ground at the scene of a car bomb attack the police college in Sanaa January 7, 2015.

A suicide bomber driving a minibus killed at least 33 people Wednesday as cadets gathered to enroll at a police academy in the heart of Yemen’s capital, Sanaa, authorities said.

No one immediately claimed responsibility for the attack. Yemen’s local al-Qaeda branch, targeted in frequent U.S. drone strikes in the country, has carried out similar attacks in the past.

At the scene of the blast, the dead and wounded lay on a sidewalk against a wall. Water sprayed by firefighters to extinguish the blaze mixed with their pooled blood. A charred taxi cab smoked near what remained of the minibus, meters (yards) from a gate for the police academy, located in a residential area.

The bomber struck as lines of cadets waited outside of the academy, preparing to enroll, witnesses said.

“We were all gathering and … (the bomber) exploded right next to all of the police college classmates,” eyewitness Jamil al-Khaleedi told The Associated Press. “It went off among all of them, and they flew through the air.”

The head of police in Sanaa, Abdul-Razak al-Moayed, said the bomber killed at least 33 people. Another security official said at least three civilians died in the blast. The official spoke on condition of anonymity as he wasn’t authorized to brief journalists.

There has been a hike in violence in Yemen recently after Shiite rebel militias took control of the capital and other cities.

The rebels, known as Houthis, are challenging the government of President Abed Rabbo Mansour Hadi, demanding a greater share of power. The critics of the Shiite rebels view them as a proxy for Shiite Iran, charges the rebels deny.

Tribal leaders and Yemeni officials warn the rising power of the Shiite Houthis and the backlash over drone strikes has caused al-Qaeda to surge in strength and find new recruits. The Houthis’ push into largely Sunni regions of central Yemen has pitted the rebels against Sunnis, to the benefit of Sunni al-Qaeda.

Washington considers al-Qaeda in Yemen, linked to several failed attacks in the U.S., the most dangerous branch of the terror network.

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