BEIRUT – The early-morning barrage against rebel-held areas around the Syrian capital immediately seemed different this time: The rockets made a strange, whistling noise.

Seconds after one hit near his home west of Damascus, Qusai Zakarya says, he couldn’t breathe, and he desperately punched himself in the chest to get air.

Meanwhile, in rebel-held areas east of Damascus, hundreds of suffocating, twitching victims were flooding into makeshift hospitals following a similar rocket barrage. Others were later found dead in their homes, towels still on their faces from their last moments trying to protect themselves.

In a series of interviews with The Associated Press after the suspected poison-gas attack on Aug. 21, witnesses, survivors and doctors described scenes of horror they say will haunt them forever.

Activists and the group Doctors Without Borders say at least 355 people died in the attack that has provoked international condemnation and shocked a world that had grown largely numb to the carnage of Syria’s civil war, which has killed more than 100,000 people in 2½ years. Fueling the outrage were online videos that showed scores of children killed in the attack.

Convinced that President Bashar Assad’s regime was responsible for the attack — a charge Syrian officials strongly deny — the U.S. and its allies are now hurtling toward military action, though they have not yet presented concrete proof.

U.N. chemical weapons experts this week took biological samples from several victims — a step U.S. officials said came too late. But they are not seeking to answer the question of who was responsible for the attack, just whether chemical agents were involved.

Witnesses interviewed by the AP say they can’t prove it but strongly believe government forces were responsible, saying that it is consistent with the nature of Assad’s regime and that nobody else had the capability to fire such weapons.

The U.S. administration, meanwhile, is said to be preparing a report for key members of Congress laying out the evidence against the Assad government. A declassified version was to be released to the public, but so far that has not happened.

“To suggest that the rebels did it is simply ridiculous. … Why would they hit themselves with chemicals?” asked Ammar, a resident who said he miraculously survived the barrage on Moadamiyeh, where 80 people were killed. He declined to give his full name because he was afraid for his life.

The rocket assaults came around the same time on two suburbs on opposite sides of the capital: Moadamiyeh to the west and several districts to the east, including Zamalka, Ein Tarma and Arbeen. The two areas are around 10 miles a part.

Ammar said he was awakened by shelling around 5 a.m., just before dawn prayers, when he heard a screeching sound unlike anything he had heard before, followed by the sound of people screaming on Rawda street below his apartment. Once outside, he said, he saw a gas with a faint green color. It “stung my eyes like needles.”

“I ran out to see what was going on and saw people in various stages of suffocation and convulsions. I tried to help, but then my legs buckled and I fell to the ground,” he said.

Ammar woke up at a makeshift hospital, previously a Red Crescent center, where he said he spent five days getting water, oxygen and injections of atropine, which can be used to counteract the effects of nerve gases.

A week later, Ammar said he has not fully recovered. He suffers bouts of cold sweats, exhaustion, hallucinations and a runny nose. Worst of all, he said, were the nightmares.

“I can’t sleep anymore. I keep seeing the people who died, the scenes from the hospital of people twitching and foaming. I can never forget that,” said Ammar, 30, who worked in the clothing business before the war and now is a government opponent who sometimes deals with the media.

His father, who identified himself by his nickname, Abu Ammar (Arabic for Father of Ammar), was at the nearby al-Rawda mosque along with a small group of people waiting for dawn prayers when the first rockets hit. He said some people ran outside and then came back in immediately, shouting, “Chemicals! Chemicals!”

He put water on a tissue and covered his mouth and nose, and then went out.

“I saw at least seven people lying on their backs, completely still,” he said.

Zakarya said the rockets crashed with a strange whistle “like a siren.” Friends took him to the hospital, where he saw dozens of people crowding the rooms and corridors, many of them in their underwear as nurses and doctors doused them with water. That was when he fainted.

When he came to, doctors were injecting him with atropine and he started vomiting. “Strange colors came out of my stomach,” the man said. He fainted again and later woke up in the street outside in his underwear, apparently moved out to make room for others.

Later, he felt well enough to go home and said he slept for 13 hours. “When I woke up I felt like Alice in Wonderland,” he said. “Everything looked distorted and I couldn’t remember anything.”

To the east of Damascus, some 600 patients poured into a makeshift hospital. in the district of Arbeen, most of them from the nearby Zamalka area, said Abu Akram, a 32-year-old doctor at the facility. Of those, 125 died, including 35 children, he said.

He said the signs — twitching, foaming at the mouth and nose, constricted pupils — were all clear signs of a kind of nerve gas.