BENGHAZI, Libya – Heavily armed militants used a protest of an anti-Islam film as a cover and may have had help from inside Libyan security in their deadly attack on the U.S. Consulate, a senior Libyan official said Thursday.

As Libya announced the first four arrests, the clearest picture yet emerged of a two-pronged assault with militants screaming “God is great!” as they scaled the consulate’s outer walls and descended on the compound’s main building.

The rampage killed the U.S. ambassador and three other Americans.

Eastern Libya’s deputy interior minister, Wanis el-Sharef, said a mob first stormed the consulate Tuesday night and then, hours later, raided a safe house in the compound just as U.S. and Libyan security arrived to evacuate the staff. That suggested, el-Sharef said, that infiltrators within the security forces may have tipped off the militants to the safe house’s location.

The attacks were suspected to have been timed to coincide with the 11th anniversary of the Sept. 11, 2001, terrorist strike in the United States, el-Sharef added, with the militants using the film protest by Libyan civilians to mask their action.

Killed in the attack were U.S. Ambassador Chris Stevens, information management officer Sean Smith, private security guard Glen Doherty and one other American who has yet to be identified.

El-Sharef said four people were arrested at their homes Thursday, but he refused to give any further details. He said it was too early to say whether the suspects belonged to a particular group or what their motive was. Libya’s new prime minister, Mustafa Abu-Shakour, said authorities were looking for more suspects.

One of five private security guards at the consulate said the surprise attack began around 9:30 p.m. when several grenades that were lobbed over the outer wall exploded in the compound and bullets rained down.

The guard was wounded in the left leg from shrapnel. He said he was lying on the ground, bleeding and in pain when a bearded gunman came down the wall and shot him twice in the right leg, screaming: “You infidel, you are defending infidels!”

“Later, someone asked me who I was. I said I was the gardener and then I passed out. I woke up in hospital,” said the guard, who spoke to The Associated Press from his bed at a Benghazi hospital on condition of anonymity because he feared reprisals and reprimands from his employers.

The witness account came as protests of the obscure film, “Innocence of Muslims,” continued in the Middle East.

An angry throng broke into the U.S. Embassy in Yemen, and clashes between security forces and demonstrators near the fortress-like embassy compound in Egypt in the heart of Cairo left nearly 200 people injured and two police trucks burned.

In Yemen’s capital of Sanaa, hundreds of protesters chanting “death to America” and “death to Israel” stormed the U.S. Embassy compound and burned the American flag on Thursday.

Yemen’s president, Abed Rabbo Mansour Hadi, apologized to the U.S. and vowed to track down the culprits, just as Libya’s president did. Egypt’s Islamist President Mohammad Morsi, who had been slow to speak out on Tuesday’s assault on the embassy in Cairo, promised Thursday that his government would not allow attacks on diplomatic missions.

The crowd in Sanaa swarmed over the embassy’s entrance gate.

Inside the compound grounds, they brought down the American flag in the courtyard and replaced it with a black banner bearing Islam’s declaration of faith – “There is no God but Allah.” They did not enter the main building housing the embassy’s offices. Demonstrators set tires ablaze and pelted the compound with rocks.

Witnesses said the protesters set ablaze a room housing security guards and torched several cars.

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