The police investigation into the Agate Pass accident that sent two drivers to the hospital with serious injuries and caused an island-wide traffic jam in early March has been wrapped up.

Bainbridge Island police said Tuesday the collision was caused by a distracted driver. Even so, some bit of mystery remains.

While the investigation has ended, the exact cause of the crash remains unknown.

“We have not located a specific cause,” said Bainbridge Police Lieutenant Chris Jensen. “Obviously a distracted driver, but we don’t know what caused her to do it.”

The crash closed down Highway 305 for nearly four hours on March 4, trapping drivers on both sides of the Agate Pass Bridge and sending commuters exiting the ferry straight into gridlock.

The accident occurred around 2 p.m. when a southbound vehicle crossed the center line on Highway 305 shortly after crossing over the Agate Pass Bridge. The vehicle collided with a car traveling northbound, causing it to lift into the air, spin around and land on a guardrail. Both drivers were rushed to Harborview Medical Center in Seattle with serious injuries.

Bainbridge police are still awaiting the results of blood tests on the drivers, but they do not expect that the tests will contribute any significant information to the investigation.