Workers install protective scaffolding over the entrance to a building at 8300 Wisconsin Ave. in Bethesda on Aug. 30, 2019, a week after a glass railing shattered and fell to the sidewalk from a ninth-floor balcony, injuring a 12-year-old girl. (Katherine Shaver/The Washington Post)

Montgomery County officials are trying to figure out why glass panels on relatively new mid- and high-rise buildings in downtown Bethesda are spontaneously shattering, raining glass onto sidewalks below.

County building officials say they are investigating whether there are any connections between windows and large glass panels that have fallen over the past two years from three buildings, all built since 2014, within a half-mile radius of the Bethesda Metro station. Two of the building owners say their engineering consultants have found that the failed glass had manufacturing flaws.

In an Aug. 22 incident, a 12-year-old girl standing outside the entrance of a Harris Teeter grocery store suffered minor injuries when glass the size of pebbles fell from the balcony railing of a ninth-floor apartment. Most recently, a glass canopy at the same building, the Flats 8300 apartments at 8300 Wisconsin Ave., was discovered shattered Tuesday above the front entrance, along the same sidewalk as a school bus stop. Although the glass shattered, it did not fall.

On July 7, part of a glass railing fell from an eighth-floor balcony of the 17-story Cheval Bethesda luxury condominiums at 4960 Fairmont Ave., at the corner of Old Georgetown Road.

Reports of falling glass first surfaced in spring 2017 at an office building at 4500 East-West Hwy. Three panels of the glass facade subsequently shattered and fell to the sidewalk, while eight other panels broke but did not fall. In May, the building owner, D.C.-based Carr Properties, agreed as part of a county inquiry to replace the type of glass panels found to have manufacturing defects.

It’s unclear whether the glass was all made by the same company because the owners of all three buildings declined to provide the name of their glass manufacturers to The Washington Post.

“We need to get more answers,” said Hadi Mansouri, acting chief of Montgomery’s Department of Permitting Services. “Why all of a sudden it’s happening is something we’re trying to figure out. . . . It’s a life and safety issue.”

Mansouri said the county hasn’t determined who manufactured the glass.

Officials in Washington, Prince George’s County and Northern Virginia said they couldn’t recall any similar glass failures in recent years, and a search of international news coverage finds them to be relatively rare. However, the Bethesda incidents are similar to a case in Baltimore, where two people were injured in October after glass fell from a broken eighth-floor window in the 20-story Exelon Building downtown, according to news media reports.

A glass panel at the entrance to the building at 8300 Wisconsin Ave. was found shattered Sept. 3, 2019. Montgomery County officials are investigating whether there are any connections between three shattered-glass incidents in downtown Bethesda in recent months. (Amanda Farber)

The Bethesda cases have alarmed residents in an inner suburb undergoing a boom in high-rise construction, including several buildings with glass facades.

“What are the odds that you’d have three buildings in one downtown area that all have issues?” said Bethesda resident Amanda Farber, who has been tracking the problem. “I’d like to know what the common denominator is. They’ve all happened under different weather conditions and at different times of the year. Is it a defect with the glass itself?”

Paige Coulman, another Bethesda resident, said she saw the pellets of glass rain down Aug. 22 on a group of about four adults and four children outside the Flats 8300 apartment building.

Coulman, who was driving by, said she saw the group cover their heads and run across the street. On Tuesday morning, Coulman said, she was startled to see the shattered glass canopy at another entrance to the same building.

“I thought, what’s going on?” Coulman said. “It’s scary.”

In all of the incidents, Mansouri said, the glass fell in small pellets rather than in larger, more dangerous shards because the glass was tempered, as required by the international building code.

All three buildings now have scaffolding and overhead canopies protecting sidewalks below, some ordered by the county and some installed voluntarily by the building owners.

In the Baltimore case, the building owner, Baltimore-based Beatty Development Group, found probable manufacturing impurities in multiple glass panels, according to media accounts. A spokesman for the company was unavailable this week.

A consultant for Carr found the same flaw — the presence of nickel sulfide — in glass that fell from the facade at 4500 East-West Hwy. When the glass production process is contaminated, tiny balls of nickel sulfide can form in the glass and, over time, cause the glass to crack or break without warning, according to experts.

A consultant for Duball Fairmont, the Reston-based developer of the Cheval condo building, also found nickel sulfide as the likely cause of the balcony railing’s July 7 break, according to a developer’s memo sent to building residents.

One glass expert said finding out which company or companies manufactured the problematic glass will be key, particularly if nickel sulfide is found as the root cause in all three buildings.

George Dotzler, whose Miami company, Construction Research Laboratory, tests products used in building facades, said problems due to nickel sulfide were more common in the 1960s. However, he said, he’s found them “terribly infrequently” since the 1970s, when U.S. glass manufacturers found ways to ensure nickel sulfide wasn’t left behind.

The shattered panel at 8300 Wisconsin Ave. on Sept. 3. (Amanda Farber)

But Dotzler said he’s seen more cases in the past five to six years, all in glass manufactured by Chinese firms, often at a lower price.

“It really doesn’t happen with European or North American or even South American [glass] manufacturers,” Dotzler said.

Dotzler, who emphasized that he had no knowledge of the Bethesda cases, said he would also want to know why the balcony glass fell after shattering.

“A properly designed railing, even if the grass breaks, shouldn’t fall,” Dotzler said.

The owners of all three buildings declined interviews but released statements.

Carr, the owner of the office building at 4500 East-West Hwy., said it expects to replace the problematic glass over the next year.

Marc Dubick, president of Duball, provided a detailed memo sent to Cheval residents about the broken balcony railing. The developer is replacing all of the building’s glass railings “out of an abundance of caution,” the memo said.

Milton Schwarz, president of the Cheval condominium board, said he’s pleased with the company’s response.

“These things happen,” Schwarz said.

Greenbelt-based Bozzuto, which manages the Flats 8300 apartments, said that in light of the shattered glass canopy discovered Tuesday, it is extending the building’s protective scaffolding “as an added precaution” — something Mansouri said the county is also considering as an order.

“We have been working closely with building engineers and materials scientists to diagnose the issue and investigate further,” Bozzuto said in a statement.

The firm also has been in touch with the family of the injured 12-year-old girl, according to the statement.

“Our company was founded on the values of care and concern,” Bozzuto said. “As the property manager, the safety of our residents and everyone who visits the community is our highest priority.”