The incident occurred last weekend at the National Nuclear Security Administra­tion’s Y-12 complex in Oak Ridge, Tenn., a major hub of the U.S. nuclear weapons program whose long history includes enriching uranium for the bomb dropped in Hiroshima on Aug. 6, 1945.

The protesters used bolt cutters to open fences and made their way to the plant’s most secure area, where warheads are serviced and where the nation’s bomb-grade uranium is stored, according to The Associated Press.

The intruders attached banners to the building, splashed it with human blood and spray-painted slogans before they were arrested.

An investigation revealed unspecified security flaws that led to a “temporary standdown” Wednesday to allow for refresher training and instruction.

Savannah River Site has no plans to halt any of its operations, but it is reviewing its procedures to make sure a similar breach could not occur there, said Bill Taylor, a U.S. Department of Energy spokesman.

“The Savannah River Site reviews security procedures whenever an event occurs at another site,” Taylor said. “The site will also participate in any lessons-learned activities with other DOE sites.”

SRS and Y-12 use the same security contractor, WSI – formerly Wackenhut Services Inc. – whose parent company is now called G4S Government Solutions.