A massive explosion of military munitions shook San Antonio in late 1963, generating a mushroom cloud, breaking windows up to 30 miles away and briefly raising concerns about nuclear fallout.

The huge blast in southwestern Bexar County, which surprisingly injured no one seriously, was overshadowed by news nine days later when President John F. Kennedy was assassinated in Dallas, one day after visiting the Alamo City.

Official accounts indicate 123,000 pounds of explosives accidentally detonated as three workers took similar material to a fortified bunker at Medina Base, then an Atomic Energy Commission site, now known as the Lackland Training Annex.

“I guess that was about the scariest situation I’ve ever been in,” recalled one worker, Floyd Lutz, now 75.

The men had only seconds to escape after spotting a fire — the cause of which wasn’t determined — in the thick-walled bunker called an “igloo.”

“We were running straight away and the blast felt like it shoved us down the road an additional 50 yards,” recalled Lutz of San Antonio. “We didn’t know if we’d exist or not when it detonated.”

Cars and planes, equipped with sensors to detect radiation quickly, arrived to determine if nuclear material had been released, according to press reports, which indicate no abnormal radiation was found.

But that didn’t end public speculation and suspicions that were fed by the limited government disclosures about the explosion that rattled both buildings and nerves.

“Much confusion surrounded the blast, some thinking it was a nuclear explosion,” Donald R. Barnes said in a 2010 online posting in which he identified himself as working as a security staffer that day at Medina Base. “The information regarding the blast hit the airwaves and appeared to travel all over the U.S. in a matter of minutes. It was reported that city evacuations were underway. A lot of misinformation was reported.”

Local papers featured front-page photos of the towering smoke cloud and the resulting crater, along with accounts from Lutz, Louis Ehlinger Sr. and Hilary Huser, all employees of Mason & Hanger — Silas Mason Co. (now American Ordnance LLC).

Their job — then secret — was to dismantle Cold War weaponry and to either burn the non-nuclear components or store the explosives in 90 igloos there that were designed to force any blasts downward.

“A lot of people suspected what was going on there but nobody really talked about it,” said Huser, now 80 and a Blanco resident. “We were inspecting and retiring atomic weapons.”

The veil of secrecy was officially lifted in 2001 when Medina Base appeared on a government list of 317 sites nationwide that handled nuclear materials during the Cold War.

The announcement stemmed from a government program established to cover the medical costs, and provide other compensation, to employees made ill from handling nuclear materials.

Lutz, Elinger and Huser say they suffered no such maladies.

On the day in question, Huser recalled taking inventory of explosives aboard a large transport vehicle called a “straddle-carrier” when a popping sound — similar to a shotgun discharging — emitted from inside the igloo where Lutz and Ehlinger were working.

“The hair stood up on the back of my neck and, right away, Louis came running out the door and hollered at me to run. He didn’t have to tell me,” said Huser, who made it about 100 yards before the blast went off, damaging his left eardrum.

Then debris came raining down and, through the smoke and dust, he noticed two straddle-carriers that were near the igloo had vanished, said Huser.

He feared Lutz and Ehlinger were dead until an ambulance later pulled up with them inside.

After being checked by doctors, the men faced a barrage of questions from military officials, police and the media.

“The FBI interviewed us individually to see if any crime was involved and if our stories meshed,” recalled Huser. “Generals from Lawrence Radiation Laboratories in California wanted to know what happened. There was a doctor who said he never had had any live guinea pigs before who’d been in an explosion like that and survived.”

“We had all the attention until President Kennedy was assassinated,” he said. “As far as the news went, we were old news then.”

Ehlinger, now 87, believes negative publicity over the blast prompted the DOE to relocate its operations from Medina Base, where it employed an estimated 500 people between 1958 and 1966, to a Pantex site outside Amarillo.

“It scared the hell out of a lot of people,” he said of the blast from his Devine home.

As he fled the igloo, Ehlinger hid in a drain pipe for protection, but a flying piece of concrete hit him on the leg, he said, showing off a small knot that persists on his thigh.

The array of igloos is still visible to motorists on Loop 1604 south of U.S. 90, said Huser, “and there’s a blank spot where the one was that we blew up.”

“No telling what’s in them,” said Ehlinger.

The former coworkers, who haven’t spoken in years, say they rarely mention their long-ago brush with death.

“That’s old history,” said Lutz.