Montgomery County officials are investigating why a piece of glass fell Thursday from a ninth floor balcony of a Bethesda apartment building at 8300 Wisconsin Ave. (Katherine Shaver/The Washington Post)

A glass panel fell from the ninth-floor balcony of a Bethesda apartment building Thursday, injuring a girl hit by glass pellets that rained down onto a sidewalk below, Montgomery County authorities said.

The 12-year-old girl was taken to Suburban Hospital with non-life-threatening injuries, said Edward Sherburne, chief of the Bethesda-Chevy Chase Rescue Squad. A witness said paramedics applied a bandage to the girl’s head.

Sherburne said the county’s Department of Permitting Services will investigate why the rectangular panel, which was about four feet by five feet, fell from the balcony railing about 5:20 p.m. The glass came from a top-floor unit at the 8300 Flats apartment building at 8300 Wisconsin Ave. just north of downtown Bethesda.

The red-brick sidewalk at the corner of Woodmont Avenue and Battery Lane — on the building’s southwest corner — was covered with glass pellets Thursday evening. The sidewalk is at the entrance of a Harris Teeter grocery store.

Sherburne said the panel, made of safety glass, was designed to shatter in place and fall as pellets rather than as a large shard.

This is the second Bethesda building where glass has fallen recently from top floors onto sidewalks below.

In May, the owner of a nine-story office building on East-West Highway, about a half-mile away in downtown Bethesda, agreed to replace part of its glass facade after finding that manufacturing defects caused glass panels to break 11 times since April 2017. Glass pellets fell to the sidewalk below at least three times. No one was injured.

Glass experts say manufacturing imperfections can cause glass to shatter spontaneously, often after sun has hit it from a certain angle.