"FBI San Antonio is aware of the incident and investigating," FBI spokesperson Michelle Lee told CNN.
The incident took place in Texas on Friday as the campaign bus was traveling from San Antonio to Austin as part of a push to urge Biden supporters to cast their ballots on the state's last day of early voting. 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 a source familiar with the incident.
At one point they slowed the tour bus to roughly 20 mph on Interstate 35, the campaign official said. The vehicles slowed down to try to stop the bus in the middle of the highway. The source said there were nearly 100 vehicles around the campaign bus. Biden staffers were rattled by the event, the source said, though no one was hurt.
Neither Biden nor his running mate, California Sen. Kamala Harris, were on the bus. Multiple sources told CNN that Wendy Davis, a former state senator who is challenging Republican Rep. Chip Roy for Texas' 21st Congressional District, was on the bus. Davis' campaign declined to comment to CNN on Saturday.
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Staffers on the bus called 911, which eventually led to local law enforcement assisting the bus to its destination.
President Donald Trump tweeted a video of the bus incident with the words "I LOVE TEXAS!" on Saturday, and claimed at a campaign rally on Sunday that his supporters were "protecting" the bus.
"But it is something, did you see the way our people they ... you know they were protecting his bus yesterday, because they're nice," the President said.
Biden told reporters in Texas Sunday evening, "We've never had anything like this."
"At least we've never had a president who thinks it's a good thing," he said.
President Donald Trump suggested the FBI should stop investigating an incident in which his supporters were seen surrounding a Biden campaign bus in Texas, leading Democrats to cancel planned events there.
The president's tweet on Sunday came hours after the FBI confirmed that it was "aware of the incident and investigating.''
Trump retweeted a screenshot of the FBI statement, adding, "In my opinion, these patriots did nothing wrong.''
He suggested the agents should instead be investigating protest-related violence in cities run by Democratic mayors. Videos posted online on Friday showed a group of cars and pickup trucks -- many adorned with large Trump flags -- riding alongside the Biden campaign bus as it traveled from San Antonio to Austin.
This story has been updated with FBI confirmation of the investigation and comments from Joe Biden.
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