The Interior Department’s inspector general has opened an investigation into possible funding irregularities involving the proposed delta tunnels, a $15 billion plan to dig giant twin pipes to siphon water directly from the Sacramento River and send it underground to farms and cities in the southern part of the state.

The decision, made public Monday, came after a nonprofit called Public Employees for Environmental Responsibility filed a complaint alleging that federal money intended to go for fish and wildlife was spent instead on planning for the tunnels.