SANAA, Yemen (AP) — A suicide bomber driving a minibus killed at least 33 people on 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, but Yemen’s local al-Qaida branch has carried out similar assaults in the past against the army and police, viewing them as U.S. proxies. The U.S. provides counter-terrorism training and assistance to Yemeni forces, and frequently carries out drone strikes targeting al-Qaida.

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 smoldered 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 the academy, preparing to enroll, witnesses said.

“It went off among all of them, and they flew through the air,” eyewitness Jamil al-Khaleedi told The Associated Press.

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

Violence has soared in Yemen since Shiite rebels known as Houthis swept down from their northern strongholds last year, capturing various cities and the capital.

The rebels challenge the government of President Abed Rabbo Mansour Hadi, demanding a greater share of power. But they are also going after al-Qaida, with clashes and targeted attacks leaving scores dead. Critics of the rebels view them as a proxy for Shiite Iran bent on a power grab, charges the rebels deny.

Tribal leaders and Yemeni officials have said the rising power of the Houthis, their advance into Sunni areas, and the backlash over drone strikes has caused al-Qaida to surge in strength and find new recruits.

Washington considers al-Qaida’s Yemen affiliate, linked to several failed attacks in the U.S., to be the most dangerous branch of the global terror network.

Reporting by The Associated Press