NEW YORK (Reuters) - A former guard at New York City’s troubled Rikers Island jail complex brutally and repeatedly kicked an inmate in the head in 2012 while the man lay helplessly, leading directly to his death, a federal prosecutor told jurors on Friday.

Brian Coll, former Rikers Island guard, accuse of causing the death of an inmate in 2012 after repeatedly kicking him in the head, is shown in this New York City Department of Investigation photo released in New York, U.S. on November 23, 2016. Courtesy New York City Department of Investigation/Handout via REUTERS

Brian Coll, 47, sat quietly as Assistant U.S. Attorney Jeannette Vargas told a federal jury in Manhattan how the onetime correction officer had “savagely” beaten 52-year-old inmate Ronald Spear and then tried to cover up the crime.

“That’s what this case is about,” she said in her opening statement at Coll’s trial. “It’s about an officer who was put in a position of power over others and how he abused that power.”

Coll’s lawyer, Joshua Dratel, called the case a “tragedy.” But he said prosecutors lacked evidence to show the alleged kicking led to the death of Spear, who suffered from severe medical issues.

“His death was not the result of Mr. Coll’s conduct,” Dratel said.

The case is one of several recent prosecutions targeting employees at Rikers Island, one of the country’s largest correctional institutions, as authorities seek to stem violence and corruption that have long plagued it.

Mayor Bill de Blasio has sought to implement changes and increase funding at Rikers. The city’s jails house about 9,790 inmates.

U.S. Attorney Preet Bharara, whose office is prosecuting Coll, previously sued the city over a pattern of violence against juvenile inmates, a case that ended after the city agreed to several reforms.

Coll, who faces up to life in prison if convicted, was arrested in June 2015, nearly a year after the city agreed to pay $2.75 million to Spear’s family to settle litigation over his death.

According to prosecutors, on Dec. 19, 2012, Spear, who had been suffering from end-stage renal disease, sought to see an on-duty doctor but was stopped by Coll, who said he was not available.

Vargas said Spear then kicked Coll, leading the officer to punch Spear and two other officers to restrain the inmate on the ground. Rather than stop there, Coll then repeatedly kicked Spear in the head, she said.

Spear was pronounced dead shortly afterwards, prosecutors said. A medical examiner later ruled the death a homicide.

The other two guards, Brian Taylor and Anthony Torres, have pleaded guilty to their roles in a cover-up prosecutors say Coll led. Torres is expected to testify as a cooperating witness.