TransAsia Airways have made its pilots who operate the airline's ATR propeller-jets take proficiency tests Saturday, three days after one of the carrier's ATRs crashed into a river, killing at least 39 people.

The airline had canceled 90 flights over the next three days to accommodate the requirement by Taiwan's Civil Aeronautics Administration that 71 of its ATR pilots be retested.

Preliminary investigations indicate the pilots of Wednesdays' crash shut off a running engine of the ATR 72 after its other engine went idle. Aviation experts say the move was an error.

"It's a mistake," said John M. Cox, a former US Airways pilot and now head of a safety-consulting company. "There are procedures that pilots go through -- safeguards -- when you're going to shut down an engine, particularly close to the ground. Why that didn't occur here, I don't know."

Local prosecutors have said they will be looking into the possibility of "professional error."

Thomas Wang, head of Taiwan's Aviation Safety Council, said Saturday that it was too early to reach conclusions about any pilot error.

The crash into the Keelung River in Taipei minutes after takeoff killed at least 39 people and left four missing. Fifteen people were rescued after the accident, which was captured on a dashboard camera that showed the plane banking steeply and scraping a highway overpass before it plunged into the water.

The Associated Press contributed to this report