Washington (CNN)The FBI is investigating the alleged harassment of a Joe Biden campaign bus last week by motorists displaying Trump 2020 flags, a law enforcement source familiar with the probe told CNN on Sunday.

A Biden campaign official described the motorists' actions as an attempt to slow down the bus and run it off the road. People in vehicles that were part of a "Trump Train" began yelling profanities and obscenities and then blockaded the entire Biden entourage, according to another source familiar with the incident.