Wes Mann heard a spectacular thump yesterday morning and began to wonder if a large machine had fallen on the higher floor of the Bethesda building where he works.

"It was big. Almost like an earthquake," said Mann, 22, a sound engineer for Potomac Talking Book Services. "Honestly, it sounded like someone dropped something huge."

About 11 yesterday, a crane hoisting air-conditioning equipment to the roof of a building near Elm Street and Arlington Road in Bethesda overextended, lost balance and collapsed. It landed heavily on the roof of the building where the cooling equipment was to go and on stores directly beneath the crane's span. The crane was extended over the stores to service the adjacent building.

Emergency officials at first feared that people in the building at 4940 Hampden Lane were trapped inside, but that wasn't the case. No one was injured, said Montgomery County Fire and Rescue spokesman Pete Piringer.

Stores in the Shoppes at Bethesda and several nearby businesses were closed as parts of downtown were cleared. Between 50 and 60 people were evacuated. "We just stopped and asked, 'What the heck is going on?' " Mann said. "Everybody was coming out of the office because there was a smell of gas."

The crane caused a gas leak on the roof, but it was quickly contained, Piringer said.

Building inspectors tested the structures yesterday afternoon. Several cranes were brought in to right the machine. The accident drew about 75 emergency workers, along with officials from Maryland's Division of Labor and Industry.

The crane is owned by Digging & Rigging Inc. of Clarksburg. "We're just making it safe, that's all," said a company employee at the accident scene who would not identify himself.

After the collapse, building customers, employees and people in the vicinity ignored the drizzle and converged, curious about what had caused the crashing sound and prompted the abundance of yellow emergency tape in Bethesda's streets. Authorities closed or partially restricted several streets around the crane collapse, snarling traffic in the bustling area.

More than 100 people looked on. Some flashed their cell phone cameras and even posed with the crane in the background -- stuck awkwardly at about a 45-degree angle.

Nelson Manso, 61, a security employee for the building where the crane was to put the cooling equipment, said he saw the accident.

He said the crane's arm was fully extended as it lifted the equipment. The crane's base began to shake and soon collapsed. Manson, originally from Colombia, called the impact of the collapse impressive.

"It sounded like a gunshot," he said in Spanish.

Mann, who went to a nearby garage to get an overhead view of the damage, said the crane crushed the air-conditioning unit and caused damage to the roof of the shops and the office building where he works.

Piringer said the crane came down slowly enough that damage was mostly superficial.

Despite the chaotic scene, the crowd of spectators and people evacuated did not panic, Mann said.

"Everybody was really calm," he said. "If anything, it was more novelty."

Workers talk near the site of a crane accident on Hampden Lane in Bethesda, where a crane hoisting machinery toppled and struck the roof of a building in the Shoppes at Bethesda.