On Wednesday, the defense team called a retired U.S. Bureau of Prisons warden to describe the extreme isolation that Tsarnaev would face at the 490-cell prison in the barren foothills of Colorado’s Rocky Mountains.

The former warden, Mark Bezy, described the ADX as “the most restrictive penitentiary in the federal system,” and said Tsarnaev would likely be placed in a unit reserved for terrorists whose contact with other people is severely limited. His testimony was expected to continue Thursday.

Even in general population, the ADX imposes extreme isolation. Prisoners spend about 23 hours a day in solitary confinement in 12-by-7-foot cells with a single 4-inch-wide window and walls thick enough to stifle any attempts at communication. A slot in the door is used to deliver meals and for any visits.