(Reuters) - Two sheriff's deputies were shot dead and two others wounded early on Thursday in LaPlace, Louisiana, a spokeswoman for the state police said.

The officers were providing security at an off-site parking lot used by contractors at the Valero Energy Corp St. Charles refinery.

State police spokeswoman Melissa Matey told CNN that two of the St. John the Baptist Parish deputies were working off-duty and the other two were on duty.

"They had an encounter, we're not sure they were ambushed," Matey said. "We are in the early stages of investigation."

Two suspects are in custody and a third is being sought, she said.

LaPlace is about 25 miles west of New Orleans. Valero said in a statement that refinery operations were not affected.