HO CHI MINH CITY, Vietnam - As the Marines scrambled to the roof of the U.S. Embassy, they locked a chain-link gate on every other floor to slow the throng of panicked Vietnamese civilians sure to come behind them. They knew if the crowd pushed through to the top, they could easily be overrun by hundreds of people desperate to get a seat on one of the last helicopters leaving Saigon.

The men barricaded the rooftop door using heavy fire extinguishers and wall lockers and waited nervously as Vietnamese outside rammed a fire truck through an embassy entrance. They could hear looting going on below and watched as cars were driven away and everything from couch cushions to refrigerators was carted out of the offices. They saw South Vietnamese soldiers strip off their uniforms and throw them into the street, fearing they would be shot on sight by the northern enemy.

It was still dark when the U.S. ambassador left the roof on a helicopter around 5 a.m. April 30, 1975. A message went out over the radio with his code name, "Tiger, Tiger, Tiger," followed by "Tiger out," to signal that the diplomat was en route to safety.

As the sun came up, the remaining Marines realized they had been forgotten. The pilots mistakenly believed that the call meant everyone had been evacuated. The Marines had no way to contact U.S. airmen ferrying Vietnamese allies and Americans to aircraft carriers offshore because their radio signals didn't carry that far.

The last U.S. servicemen in Vietnam were stuck alone atop the embassy, hoping someone would realize they were there before the city fell to rapidly advancing communist forces.

On the 40th anniversary of the fall of Saigon on Thursday, a group of Marines who were there that day returned to what is now Ho Chi Minh City for a memorial ceremony at the site of the old embassy, which is now the U.S. Consulate. They had been in charge of guarding the facility and the defense attache office beside Tan Son Nhut airport, and were tasked with helping to get the last Americans out.

The days leading up to the end of the Vietnam War were chaotic and exhausting. Northern enemy forces had been sweeping southward for weeks, capturing major South Vietnamese strongholds as they went. Everyone knew it was only a matter of time before the capital, Saigon, also fell. Rumors of a looming bloodbath gripped the city, and Americans along with their South Vietnamese allies were being evacuated on cargo planes from the airport.

Lance Cpl. John Stewart, now 58, of Nacogdoches, Texas, was assigned to take a bus through Saigon to pick up those eligible to leave. He was just 18 and had only been in the country a couple weeks, but he saw anger growing among those on the streets as people realized the end was near and the U.S. was pulling out its last remaining citizens. At one point a rocket hit near the bus and it was shaken by shrapnel, but no one was injured and the evacuation continued.

"We were having to pull people off or physically keep them from getting onto the buses," he said. "You couldn't blame them for wanting to get out, but it had gotten to the point where we could only take those that we absolutely had to. We couldn't take everybody. That's when your brain tells you this is really happening and we've reached the end, and hopefully we'll get out before the end gets here."

Stewart and the others were already shaken after a rocket attack killed two Marines - Cpl. Charles McMahon and Lance Cpl. Darwin Judge - early April 29 while they stood guard at the defense attache compound. They would be the last U.S. servicemen to die in a war that killed some 58,000 Americans, up to 250,000 South Vietnamese allies and an estimated 3 million North Vietnamese and civilians.

Thirteen of the original Marines on Thursday placed red roses in front of a plaque honoring them, saluting it on the old embassy grounds as "Taps" played.

The bodies of McMahon and Judge were found by Sgt. Kevin Maloney, now 62 of Hollywood, Fla. But there was no time to mourn. Like Stewart, he had to help load buses.

While in the city, he locked eyes with a little boy with light brown hair. He was only supposed to pick up Americans, but he shoved the kid and his mother into one of the front seats anyway, knowing that the child was most likely the son of a GI. He still wonders whether they made it onto a plane and on to America, as many Vietnamese did after being evacuated.

Once the airport became too bombed out to continue operations there, helicopters were ordered to land at the embassy for the final flights.