HOUMA, La. (AP) — Authorities planned to search a southern Louisiana marsh by boat Monday for clues that might determine the cause of a helicopter crash that killed eight people bound for an offshore oil platform.

The helicopter, operated by PHI Inc., crashed Sunday afternoon shortly after taking off in southern Louisiana, said Richard Rovinelli, a company spokesman. Two pilots and seven passengers were aboard when the helicopter went down near Houma, about 100 miles southwest of New Orleans.

The identities of the victims were not released.

Ted Lopatkiewicz,...