The guards, authorities say, wanted to send a message.

"Somebody's leaving in an ambulance tonight," Eliseo Perez, an assistant chief for security at New York's Rikers Island jail complex, told inmates after a rash of attacks on guards, according to prosecutors.

Then he allegedly instructed five subordinates to take prisoner Jahmal Lightfoot into a room and kick his teeth in, an attack that left him with facial fractures.

The ongoing trial of nine correction officers for the alleged 2012 assault and a subsequent cover-up is the latest in a string of prosecutions targeting dozens of Rikers employees over the past four years.

More than 50 guards at the 10,000-inmate complex, one of the three largest in the United States by population, have faced criminal charges since 2012 for assault, falsifying reports and smuggling contraband, court documents and data from various city agencies show.

That is about double the rate of prosecution in the prior four years, as authorities crack down on what they say is a toxic atmosphere of violence and corruption.

"Rikers is a very troubled institution," said Mark Peters, the commissioner of the city's Department of Investigation, which leads most Rikers-related probes. "We are now seeing the result of systemic neglect."

Rikers houses male, female and adolescent prisoners in 10 separate facilities, mostly inmates awaiting trial.

Rikers Under Microscope

Mayor Bill de Blasio has made Rikers reform a priority since taking office in 2014.

Peters, whom de Blasio appointed two years ago, said he had devoted one investigative squad exclusively to Rikers and increased its staff from 20 to 30 members.

Bronx District Attorney Darcel Clark, whose office prosecutes most Rikers-related cases, recently proposed a new prosecution bureau based at the complex itself.

The list of law enforcement officials whose attention has turned to Rikers also includes Preet Bharara, the U.S. attorney in Manhattan.

Next month, federal prosecutors will put two guards on trial for the fatal beating of an inmate. Brian Coll, a correction officer, is accused of stomping Ronald Spear to death and enlisting two other guards to help him conceal the truth. One guard has already pleaded guilty to the cover-up.

Bharara's office also threw the weight of the federal government behind a lawsuit brought on behalf of adolescent inmates by the New York Civil Liberties Union. The case led to a settlement mandating reforms overseen by a court-appointed monitor.

A two-year investigation by Bharara's office found the correction department failed to discipline guards adequately for excessive force from 2011 to 2013, said Sara Shudofsky, the chief of the office's civil division, in an interview.

In 2014, de Blasio also appointed Joseph Ponte to head the correction department. Since then, it has pursued internal investigations more aggressively, with 200 cases ending in disciplinary charges last year, up from 93 in 2013, according to department statistics.

The focus on guards has met stiff resistance from the correction officers union, which claims the effort hides the true causes of Rikers' problems.

"It is clear from the department's own statistics that inmates are attacking correction officers and other inmates at an alarming rate," the union president, Norman Seabrook, said in a statement.

Seabrook also said visitors, not guards, are primarily responsible for smuggling contraband, citing the hundreds of visitors arrested in the last year for bringing illegal items into Rikers.

Smuggling Surge 

The Department of Investigation has also pursued broader reforms in response to the persistent problems, Peters said.

In 2014, an undercover investigator posing as a guard gained access to Rikers six times despite carrying heroin, marijuana and razor blades. Another department probe uncovered red flags among two-thirds of a class of new hires, such as prior felony convictions or known gang ties.

In response, the correction department has installed drug-sniffing dogs at Rikers' entrances and increased its "applicant investigation unit" from 19 employees to 87, to screen potential recruits' backgrounds and psychological fitness.

Both Peters and Ponte say their departments are now working more closely to address misconduct. That cooperation was on display this month, when a guard was caught on video assaulting an inmate who had thrown a cup of liquid in his direction.

Correction officials turned over the video to investigators immediately, and the guard was arrested within hours.

But the level of violence still troubles observers. The Lightfoot trial, which began in March, highlights the difficulties in curbing incidents by both inmates and officers.

Perez and his team were part of an elite unit assigned to reduce inmate attacks, but prosecutors say their solution was to turn to assault themselves.

Defense attorneys have argued at trial that the guards simply defended themselves when Lightfoot attacked them with a weapon. Prosecutors have said that assertion is false.

"They decided they were going to set the tone that night," Assistant District Attorney Pishoy Yacoub said at the start of the trial.