Evidence found in a rental car left in Portland, Maine, led investigators to two houses in Vero Beach, Florida. One had been rented by two brothers from Saudi Arabia.
In Vero Beach, FBI agents searched four homes in three neighborhoods, according to witnesses.
A tenant in one of those houses, Adnan Bukhari, was cooperating with the federal agents, sources said.
Bukhari's brother, Ameer Bukhari, died in a small plane crash in Florida last year, according to a lawyer for the family.