A guard at Rikers Island was ambushed by a group of inmates who beat him until he fractured his spine, officials said.
First an inmate punched him in the face, then a group of inmates began kicking and punching the officer on the ground, surveillance video shows.
The 39-year-old correctional officer was attacked at George Motchan Detention Center around 6:30 p.m. Saturday, the union sources said. The jail is scheduled to close this year as part of a plan to close the troubled jail complex.
His spine was fractured in his neck region.
"What happened to our brother last night at the hands of a violent group of cowardly inmates was sickening," Corrections Commissioner Cynthia Brann said. "They'll be held accountable for this crime, but right now our thoughts and concerns are with our hospitalized officer and his family."
She said an investigation is underway and anyone involved would be arrested.
The president of the union that represents guards said Rikers has a gang problem and held Mayor Bill de Blasio responsible for the attack.
Elias Husamudeen said he wants the Department of Justice to investigate the gangs at Rikers.
He said the inmate had previously threatened to punch the officer.
De Blasio spoke to the officer and would get to the bottom of the "heinous attack," press secretary Eric Phillips said in a tweet.
Correctional officers "have one of the hardest jobs in city government - often thankless and always dangerous," he tweeted.
The victim's cousin, Gui Souffrant, told NBC 4 New York that he had worked as a prison guard for two years.
He has a masters degree from Haiti and is married, Souffrant said.
The inmate who allegedly threw the first punch, Steven Espinal, has a lengthy rap sheet including charges of attempted murder and assaulting an officer.
It wasn't immediately clear whether Espinal had an attorney.