"It drove across the deck and up the ramp, which was in the upright position," said Sgt. Al Leblanc. "For some unknown reason, the vehicle continued on, sped up, and then it plunged into the water approximately 20 to 30 metres from shore."
Leblanc said the car was carried along by the strong current for about 200 metres, then disappeared.
"Unfortunately, someone was inside the vehicle," he said. "But that's all I can tell you. We don't have any details of how many occupants and so on."
Searchers used a fishing boat Tuesday morning to scan the water. They discovered an oil slick just after 10 a.m., said the CBC's Wendy Martin.
Martin, who is at the scene, said the dive team from Halifax went in shortly afterward to look for the small, light-coloured car.
"The current is so strong, they have scrapped the idea of going in in their zodiac, and instead they're going to go in on the fishing vessel itself," she said.
Martin said the incident happened so fast, ferry staff didn't get a chance to see if anyone else was in the vehicle.
Just last week, there was a similar tragedy in British Columbia. A woman died when her car broke through a barrier and went off a ferry at Gabriola Island.
- Vehicle speeds off B.C. ferry and sinks at Gabriola Island
In Englishtown, the RCMP said Emergency Health Services, the Baddeck and Englishtown Fire Department and the North Shore District Fire Department are helping with the search. They failed to find any sign of the vehicle Monday night.
The province's Department of Transportation has started an internal investigation into the incident.
Transportation Minister Maurice Smith said in a release they'll inspect the ferry and interview crew members to try to determine exactly what happened.
The department said the ferry went into service in 2008 and is one of the newest and busiest ferries in Nova Scotia.
It remains out of service while divers look for the car.
Englishtown is located 61 kilometres northwest of Sydney. The ferry runs across St. Anns Harbour connected to an underwater cable. It provides a shortcut to the Cabot Trail in northern Cape Breton.