The truck, worth more than â‚¬400,000 (Â£340,000) was stolen on Sunday night in the small town of Stranorlar.
It was later recovered close to the border with Northern Ireland.
The Scania fire truck, which has only been in service for a year,  was returned to the station for a technical examination.
Garda (Irish police) Insp Goretti Sheridan said vital equipment was missing from the fire engine.
"These people might have thought this was funny but it could have been a much more serious incident had there have been a fire overnight," she said.
GardaÃ­ have appealed to anyone in possession of CCTV footage that may show the incident to contact them.
The Police Service of Northern Ireland is also investigating the possibility that the fire engine may have been driven across the border.
Thieves who stole a fire engine in County Donegal may have driven the vehicle with its sirens and flashing lights switched on, police have said.