When the boom hit shortly before 10 p.m. on an autumn night a decade ago, a security guard at First Union Bank on Main Street thought robbers were breaking in.

The desk sergeant thought something exploded at police headquarters on Bedford Street.

Stamford Emergency Medical Service workers thought someone drove into their Strawberry Hill Avenue building.

At Curley's Diner downtown, the manager thought someone fell in the bathroom.

A woman who lived near Stamford Hospital thought her child pulled over a dresser.

They were all wrong. But to this day no one knows what caused the "Stamford Boom" of Oct. 3, 1999.

In fact, the mystery has grown.

The boom shook houses near Stamford Hospital, in Hubbard Heights and downtown -- and nowhere else.

Authorities ruled out earthquake, thunder, sonic boom, a gas, sewer or transformer explosion, construction blasting and meteors.

In the days that followed, it got weirder.

A man reported that he was walking on Compo Beach in Westport that afternoon when he saw a "spinning wheel in the sky" over Long Island Sound. A man on a boat off Compo Beach reported seeing a shiny object that looked like "two wheels seemingly connected, each going in the opposite direction" about the same time.

Later, about 4 p.m., a woman in Darien reported seeing "a strange spinning object" with lights before it disappeared over trees. Shortly afterward, two witnesses in Stamford reported seeing a UFO.

From Stamford, Darien and Norwalk came reports of flashing lights in the sky about 9:45 p.m. At 9:50 p.m., houses rattled in central Stamford, sending residents into the streets to ask each other, "What was that?"

Lillian Lampros of Holcomb Avenue remembers opening her door and looking into the yard.

"There was no light, no smoke, nothing," Lampros said. "It was such a big boom. I called my neighbor."

They thought a transformer exploded. They didn't think UFO.

Jon Nowinski, director of Smoking Gun Research Agency, a Westport nonprofit that investigates paranormal phenomena, said at the time that the sound could have been caused by an aircraft -- military or extraterrestrial -- that could accelerate straight up or at a steep angle fast enough to break the sound barrier. Otherwise, the sonic boom would be heard over a larger area.

"That's what struck us the most. This was isolated to a small area of Stamford," Nowinski said this week. "Another thing is that a sonic boom sounds almost like a rumble of thunder. People wouldn't think twice about it. But these people were reporting something that shocked them."

Smoking Gun researchers turned up reports of other mysterious booms elsewhere, also about a decade ago -- December 1997 near Springfield, Mo.; May 1998 near Los Angeles; August 1998 near Narragansett Bay, R.I.; and January 1999 near Denver.

Another occurred this month. Residents of Staten Island, N.Y., heard a large boom that shook buildings at 7:55 p.m. March 16. It was isolated to six neighborhoods. Authorities ruled out explosions, fireworks, sonic booms, weather and earthquake.

People often think earthquakes are the cause of rumbles, but that's unlikely in the Stamford area, where they occur once every few years and register 2.5 magnitude at the strongest, said Won-Young Kim, a seismologist at the Lamont-Doherty Earth Observatory in Palisades, N.Y.

"People do feel it -- the house shakes -- but, unless the quake is very shallow, you don't hear anything," Kim said.

Meteors are another common explanation. But you'd see those, said Andrew Ackerman, an atmospheric scientist with NASA at the Goddard Institute for Space Studies in Manhattan.

"It looks like a missile with smoke behind it. A meteor will light up a daytime sky," Ackerman said.

It's possible UFOs exist, but aircraft that can break the sound barrier are "entirely military now that we no longer have the Concord," Ackerman said. The question is "whether you can expect to get a straight answer on the comings and goings of military aircraft. This is why UFOlogy can be such a rich vein -- you don't know what is known because the military can't tell you."

Nowinski said he is "a skeptic with UFOs" and he thinks "most of the things we see can be traced to the military. They are far more advanced than they are willing to let on."

Lampros said it's possible a UFO hovered over her home a decade ago.

"I don't disbelieve it. You just never know," she said. "But 'ET' hasn't come to my house yet."

-- Angela Carella is an assistant city editor at the Advocate.