Authorities in Nevada County are investigating possible criminal negligence in the deadly avalanche near Lake Tahoe that killed at least eight skiers and guides during a backcountry trip.

The sheriff’s office has notified OSHA as part of the probe, which is examining why the excursion proceeded despite active avalanche warnings.

Blackbird Mountain Guides confirmed three of its guides died and said investigations are ongoing into the tragedy, described as California’s deadliest modern avalanche.

Six of the 15 skiers survived, while recovery efforts for the victims have been hampered by hazardous weather and ongoing avalanche danger in the Sierra Nevada, The Guardian has reported.

Officials are conducting controlled explosive mitigation to stabilize the snowpack as they continue search and recovery operations.