Investigators strongly believe the paramedic behind the wheel of an ambulance that plunged into a B.C. lake, killing her and her partner, may have fallen asleep behind the wheel, CTV News has learned.

Jo-Ann Fuller, 59, and Ivan Polivka, 65, were killed after their vehicle tumbled more than 200 feet down an embankment and dove into Kennedy Lake, on a stretch of windy highway between Ucluelet and Port Alberni on Oct. 20.

The two longtime paramedics had just transferred a patient to Port Alberni and were returning to Tofino when they were killed. Fuller had been on shift for at least 10 hours when the crash occurred.

Investigators strongly believe that Fuller, the unit chief, fell asleep and drove off the roadway while Polivka was sleeping in the passenger bay strapped to a gurney – but officials have yet to speak publically.

"At this point we are not going to be speculating on anything until we have got the full picture," said Les Fisher, Executive Director for the British Columbia Ambulance Service (BCAS).

At least four investigations were launched directly after the crash, each one starting to place blame. For decades, locals demanded improved safety measures on Highway 4, saying the often steep and curvy roadway put people in danger.

Investigators also say it's possible Fuller had a medical issue, but some paramedics are privately wondering if things might have been different if Polivka had been awake in the passenger seat.

"Our policy is that paramedics need to be able to respond to calls when they're on duty," Fisher said.

But practice is often far different in remote communities, where partners often trade off long-distance driving duties while the other sleeps in the back.

"It happens," said William (B.J.) Chute, spokesperson for the Ambulance Paramedics of B.C.

"Whether it's normal practice I couldn't speak for every single crew out there. It's certainly something that's done throughout British Columbia."

There are no rules currently preventing paramedics from sleeping on duty, something B.C. Health Minister Kevin Falcon said could soon change.

"If that indeed may have been one of the challenges behind the accident that took place, you can be sure that will be covered off pretty exhaustively in the reviews being done and recommendations will no doubt flow from that."

Investigators are now hoping the vehicle's black-box data recorder will help them piece together even more clues.

A devastating crash

Fuller and Polivka were both long-serving paramedics, with 23 and 14 years of experience, respectively. Fuller had recently become a grandmother, while Polivka had just lost his wife to cancer.

Their death shook the tiny seaside community of Tofino.

Former nurse Priscilla Lockwood rode with the pair hundreds of times, and described them as conscientious and safe.

"They were wonderful, warm people. You trusted them coming into your home at a time of great tragedy or great urgency. You knew they would give you their best and they did," Lockwood told CTV News.

With a report from CTV British Columbia's Jim Beatty