Read this passage from “Alcatraz Island” by the National Park Service. What is the implied main idea?

The US government operates federal prisons to hold people convicted of violating federal laws. Alcatraz housed some of America’s most notorious offenders from 1934 to 1963. They were held under the most secure and regimented conditions, in the virtually escape-proof environment on a rocky island in the middle of San Francisco Bay. As the highest security prison, Alcatraz kept its inmates under the tightest controls. Inmates were housed one man to a cell. Two-person cells, four-person cells, and open dormitories were common in other federal prisons. Inmates at other prisons spent most of their days out of their cells at mandatory work programs, educational classes, and recreational activities.

Prisoners were kept in isolation at Alcatraz.

The US government has many federal prisons.

Alcatraz was America’s most famous federal prison.