A police officer sprayed mace at two young men while they were handcuffed at a police station holding cell in Alton, Illinois, according to security camera footage leaked to the public, NBC's affiliate KSDK reported.

Alton Police Chief James Simmons said the encounter, which involved 16- and 17-year-old men, happened late January, but only came to his attention in March.

In the video, the officer stands in the doorway of the holding cell while the two teens sit against the wall, their hands cuffed behind their backs. The officer sprays one teen — who visibly cringes after being hit — and then sprays the other.

James Gray, the head of the Alton chapter of the National Association for the Advancement of Colored People (NAACP), told KSDK: "I was astounded to see what was happening to the two kids in the police holding cell." Gray said a copy of the video was mailed to him anonymously.

The chief admitted the investigation was not handled properly and moved too slowly.

"Obviously new procedures have to be put in place as far as the timing of when the reports are finalized and brought to my attention," Simmons said in a press conference Friday.

Alton Mayor Brant Walker told reporters that Simmons has been criticized for changes he made within the department, including cutting staff, which is why he thinks this video was released to the public.

The Illinois State Police are independently investigating the incident. Authorities did not say why the teenagers had been taken into custody, according to KSDK.

Simmons said the officer in the video has been "outstanding" since he joined the force 18 years ago.

"Obviously he made a huge mistake, an error in judgement, and because of that his actions will be looked at and possible discipline will be handed down," the chief said.