The portrait that emerged from the report on Rikers Island by the United States attorney’s office in Manhattan last week was of a place with almost medieval levels of violence, meted out with startling ferocity by guards and their superiors.

The two-and-a-half-year investigation, which focused on the abuse of teenage inmates by correction staff, was exhaustive in cataloging the brutality. But a critical question that went unaddressed is how conditions were allowed to get to this point.

Rikers has been a place of violent excess for decades. And the growing ranks of inmates with mental illnesses, reaching nearly 40 percent of the jail population today, have added to the challenges for correction officials.

But conditions worsened substantially under the administration of Mayor Michael R. Bloomberg, which reduced jail staff and failed to curb escalating violence by guards, according to former correction officials, inmates’ advocates and others intimately familiar with the jail.