Thunder Bay Police Constable Andrew Johnstone was charged with assault Friday by the province's Special Investigations Unit.

The charge against him stems from an arrest made on May 4, when 43-year-old David Ferguson was arrested by the OPP for impaired driving. Ferguson was later brought to the Thunder Bay Police station by the OPP.

The SIU says that's where their investigation revealed "a subsequent use of force" had occurred.

Earlier that evening Ferguson suffered an eye injury when taken into custody by the OPP after refusing to get out of his vehicle. However the SIU determined the use of force by the OPP officer was reasonable and legally justified.

Johnstone is schedule to appear in a Thunder Bay court room on Aug. 20.

Thunder Bay Police say Johnstone remains on-the-job, but assigned to administrative duties.