A slit in the pavilion roof allows a shaft of light to highlight the names of 11 Montgomery County residents killed on 9/11 at the time of the attacks, but the construction of the annex to the Montgomery County Judicial Center is so tall it blocks the morning light, ruining the intent of the artist that created the tribute. (Katherine Frey/THE WASHINGTON POST)

Montgomery County’s memorial to the victims of the Sept. 11 attacks was carefully designed so that on every anniversary, at precisely 8:46 a.m.— the moment the first plane hit the World Trade Center — a beam of rising sunlight will travel across a plaque, illuminating the date and the names of the 11 victims from the county.

But last week on the 11th anniversary of the attacks, the plaque was covered in the shadow of a county courthouse annex being built across the street.

County officials said Monday that the building is so tall at 114 feet that it will block light, which enters the memorial pavilion through a specially designed slit in the roof, until just past 9 a.m. Officials said the gaffe was an unfortunate oversight, but that it’s too late to change the $141 million project.

“Regrettably, we can’t alter the top of the building,” said David Dise, the top county building official.

It could have been worse for the memorial, which cost the county $100,000 and is on Courthouse Square Park in downtown Rockville. While the building blocks the light from illuminating the date of the attacks, the sun does rise high enough to start hitting the names at 9:37 a.m., the exact moment the Pentagon was hit.

Still, the artists who designed the memorial said the full impact of their work has been diminished.

“It’s a shame, the encroachment. It’s not malicious, but it’s just kind of thoughtless,” said one of the artists, Susan Flores, in a phone interview.

This isn’t the first time in recent memory that a government building has created unexpected problems in Rockville.

The state’s $53 million District Court building, which is also just across the street from the memorial, was designed with a convex set of glass windows that created a beam that scorched plants, burned passersby and earned it the nickname “death ray.” A screen was later added to block the light.

AECOM, an international engineering firm with offices in Virginia, designed the District Court building and the Circuit Court annex. The company did not return several requests for comment Monday.

County officials said the annex would add 10 new courtrooms to address increasing demands on the Circuit Court.

Since the death ray incident, county officials have been sensitive to how buildings affect sunlight, and have performed special tests to assess lighting. If they had been aware of the memorial’s light aspect, they would have addressed it in the building design, said Dise, who came to the county in 2007. But despite the numerous public meetings on the annex, no one brought up the issue, he said.

Flores said she and her husband, Gene, visited the memorial site on a cold night in 2003 to record the precise locations of certain stars to get necessary data for a Harvard astronomer, who helped determine the precise location, height and angle so that the beam would hit the plaque at 8:46 a.m. every anniversary.

Within a few months, the artists built the pavilion and 11 benches for each of the victims. They welded the victims’ favorite quotes and signatures onto the benches, and the memorial was dedicated on the second anniversary of the attacks. Flores said she hopes the county can find some way to alter the building.

In separate interviews, Scott Reilly, who first discovered the lighting issue last week, and Christine Fisher, a widow of one of the 11 victims, said they’re glad that the names on the plaque are still illuminated.

“It was saddening that an element no longer worked at the monument, but fortunately some of it still works,” said Reilly, who wrote about the lighting issue in Sunday’s Local Opinions section.

John Pontius, who is 67 and lives near the park, stood at the memorial Monday morning to pay respects. He said that if the “death ray” problem could be solved, then the current issue could be solved, too. At least he hopes so. “It was a win-win back then,” he said. “I don’t know how you make this a win-win now.”

Still, Fisher added, “I wish they would’ve figured this out before finishing the building.”