Updated

A Florida man drowned moments after killing his mother with a hammer the night before Hurricane Irma made landfall in the state, police said Wednesday.

The badly decomposed body of Lance Whatley, 40, was found on Sept. 13, days after Hurricane Irma struck the region, the Palm Beach Post reported. Whatley was reported missing after his mother, Kathie Whatley, 67, was found dead at River Park Marina on Sept. 9, the St. Lucie County Sheriff’s Office said.

Mystery surrounded their deaths for months as authorities attempted to piece together the night the mother-son duo was last seen together.

Authorities said Wednesday that Whatley, who suffered from chronic mental illness, beat his mother the night before Hurricane Irma made landfall as a Category 3 storm, the Palm Beach Post reported. He then drove his car into the North Folk of the St. Lucie River and committed suicide by drowning, investigators said.

"She would have to come home from her workplace and drive him around all night, in an effort to calm him," St. Lucie Sheriff Ken Mascara said. "From the statements we've taken, he was so excited about this storm coming -- we made the conclusion that he was just unable to be calm. They pulled over at the River Park Marina where an argument ensued, most likely due to her frustration trying to calm him from the storm and the result is that he killed her."

A hammer covered in blood was found lying next to Kathie Whatley’s body. The partially submerged car was also located nearby.

Lance Whatley’s body was discovered as the water brought in by Hurricane Irma was receding in Rivergate Park.

“The body was in a decomposing state and was also missing a head and both hands, most likely from exposure to the elements and Hurricane Irma, making identification extremely difficult,” Mascara said.