In what was the most deadly terrorist attack in Yemen in years, a suicide bomber disguised as a Yemeni soldier detonated explosives during a military parade rehearsal near the Presidential palace in Sana today, killing more than 90 people and wounding 300 others.

The casualties were mainly soldiers from the government’s central security forces, who were preparing for tomorrow’s holiday marking the 22nd anniversary of unification of north and south Yemen. Yemeni officials said defense minister Mohammed Nasser Ahmed, who was unharmed, had arrived to greet the forces minutes before the attack took place.

There are conflicting reports about who’s behind the blast. The Associated Press reported that Al Qaeda in the Arabian Peninsula (AQAP) — the branch in Yemen behind the recently foiled plot to smuggle a suicide bomber on a U.S.-bound plane — had claimed responsibility. In an e-mailed statement, AQAP said the attack was retaliation for the “crimes” of a recently-intensified Yemeni military campaign in the south — assisted by the U.S. — to retake areas that have fallen under AQAP and its affiliates’ authority over the last year. According to Al Jazeera English, a spokesperson for Ansar al Sharia, a close affiliate of AQAP, also took responsibility for the attack.

But AQAP expert Gregory D. Johnsen cautioned that it could take several days for definitive confirmation on online jihadist forums that either group is responsible.

Today’s blast follows yesterday’s news that militants in the port city of Hodeida opened fire on a vehicle and injured three American civilian contractors who were training Yemen’s Coast Guard. Pentagon spokesperson Bill Speaks told the Associated Press their injuries were minor.

“If confirmed, both attacks are demonstrating to the Yemeni government and to the US government that it can carry out strikes whenever it wants,” Johnsen told FRONTLINE. “To put someone in a military parade on what’s supposed to mark 22 years of unity is an impresive feat for Al Qaeda.”

Yemen’s new president Abdo Rabbu Mansour Hadi announced that two senior officials had been demoted for the security breach. “The war on terrorism will continue until we win, whatever the sacrifices are,” he declared in a televised address this evening.

The Nation‘s Jeremy Scahill, who’s been reporting on the escalating war in Yemen, told FRONTLINE the “boldness” and “scale” of today’s suicide attack was unprecedented. “This attack on the Yemeni military is clearly intended to send a message that the emperor has no clothes; that the US-backed Yemeni government is weak and unable to protect even its own forces,” he said. “It also reveals the strong autonomy AQAP sees itself as having, particularly after the death of bin Laden.”

The attacks come as the U.S. and Yemen escalate the military campaign in the south and a more secretive war on Al Qaeda-linked insurgents across Yemen that has included drone strikes carried out by both the CIA and the Joint Special Operations Command (JSOC), as well as more conventional air strikes.

“If it is AQAP behind the attacks, it’s their response to an intensifying U.S. war,” Scahill added. “The drones, the cruise missile attacks, and now the U.S. trainers, are a window into the future of U.S. policy in Yemen. And today’s suicide bomb and yesterday’s gun attack is a window into the future of AQAP’s policy.”