Days after a Toronto police officer was charged, Ontario's police watchdog has arrested and charged another man in connection with the serious beating of a teenager last December.

Christian Theriault, a civilian, has been charged with aggravated assault, assault with a weapon, and public mischief — the same offences faced by Const. Michael Theriault, who was charged Tuesday by Ontario's Special Investigations Unit (SIU).

This is the first time the SIU has charged a civilian, the police watchdog confirmed Friday.

Christian Theriault is set to appear in court next month.

"The allegation in this case is that Const. Michael Theriault and Christian Theriault acted together and were parties to the same assault upon the 19-year-old man on December 28, 2016," SIU director Tony Loparco said in a news release.

Michael and Christian Theriault are brothers, according to Julian Falconer, lawyer for Dafonte Miller, the 19-year-old who suffered serious injuries following a Dec. 28 incident.

According to the SIU, the two were "involved in an interaction" with Miller in the area of Thickson Road and William Stephenson Drive in Whitby.

Miller's mother, Leisa Lewis, told CBC Toronto that Miller was hit repeatedly with a steel pipe. He is traumatized and awaiting surgery to remove his eye, Lewis said.

Toronto Const. Michael Theriault is seen here in a video submitted by a CBC Toronto reader who filmed the officer while he and his partner made a traffic stop. Theriault was charged in the Miller case earlier this week. (Submitted by Joseph Briggs)

Following the December incident, Durham Regional Police arrested Miller. According to his lawyer, he was charged with theft under $5,000, two counts of assault with a weapon, possession of a weapon and possession related to marijuana, but all the charges were withdrawn following a pretrial hearing on May 5.

Miller's lawyer notified the SIU of the incident on April 27.

As protocol dictates, Const. Theriault has been suspended with pay by the Toronto Police Service.