Enriched uranium is both processed and stored at the facility, which a senior official had previously declared “the Fort Knox of uranium.” However, Peter Stockton, a former Congressional investigator and security consultant for the Energy Department, said that for years there have been questions about the building’s security, including whether guards’ sight-lines are adequate, the article reports.

The peace activists, 82-year old Sister Megan Rice, 63-year old Michael Walli and 57-year old Greg Boertje-Obed, cut through a number of fences to reach the outer walls of the “Highly Enriched Uranium Materials Storage Facility,” the government’s main stockpile site for bomb-grade uranium, the article says.

The building was designed and built after the September 11, 2001 Al Qaeda attacks with special features to withstand possible militant attacks, but Rice says that while the activists were confronted by as many as 12 guards, “they dribbled in and out … It was very gradual.” She also added: “First the one, and he began to alert others,” Reuters reports.

The activists hung banners and strung crime scene tape on the building, and daubed slogans on the outer walls, the article says. They were arrested and face charges of “willfully and maliciously destroying or attempting to destroy government property.”

As a consequence of the breach, Joshua McConaha, a spokesman for the National Nuclear Security Administration, said that the agency “has taken steps to remove the leadership team and the guard forces on duty at the time, and to replace them with some of the best security experts from around our enterprise.”