West Palm Beach, Fla. ? Authorities investigating the slaying of a family of four alongside a Florida highway said Friday that the father probably was involved in drug trafficking and that the suspected killer was in custody.

The suspect was among three men and a woman who were arrested Wednesday on drug charges and are “persons of interest” in the killings, Sheriff Ken Mascara said. None had been charged with the slayings, he said. Another man is being sought, he said.

The couple and their two young sons were found shot to death Oct. 13 off a desolate stretch of Florida’s Turnpike near Port St. Lucie, about 50 miles north of their home in the Palm Beach County city of Greenacres.

Police searching the home after the killings found evidence that the father, Jose Luis Escobedo, was involved in drug trafficking, Drug Enforcement Administration agent David Weeks said in court papers filed Friday. The items were said to include “suspected drug ledgers” and plastic packaging used to wrap illegal drugs.

Investigators linked the drug suspects to Escobedo through references in the ledgers, among other things, the DEA agent said.

A search of the West Palm Beach home where three of the men under arrest were living turned up suspected crack, Ecstasy and drug packaging materials, the agent said.

There also was a photograph of Escobedo with one of the suspects, Danny Varela, 26, according to court papers. The sheriff identified the other suspects as Liana Lopez, 18, Daniel Troya, 23, and Ricardo Sanchez, 23. The four were charged with possession of cocaine with intent to distribute.