Protesters confront with the police near an Immigration and Customs Enforcement centre in Portland, Oregon, U.S., August 20,…
Protesters confront the police near an Immigration and Customs Enforcement center in Portland, Oregon, Aug. 20, 2020, in this still image from a video obtained from social media.

PORTLAND, OREGON - Protesters in Oregon's largest city have clashed again with federal agents outside a U.S. Immigration and Customs Enforcement building that has become a new focus of the demonstrations that have gripped Portland for months, officials said Friday.

People in a group of about 100 late Thursday and before dawn Friday sprayed the building with graffiti, hurled rocks and bottles at agents and shined laser lights at them, Portland police said in a statement.

The agents set off smoke or tear gas and used crowd control munitions to try to disperse the crowd, The Oregonian/OregonLive reported. Three people were arrested, police said in their statement.

The violence came a day after protesters clashed with federal agents for the first time since July in a demonstration that also targeted the ICE building. Two people were arrested and several officers suffered minor injuries.

On Friday, a number of federal buildings across the city were closed as the FBI investigated a car bomb threat. The agency said in a statement Friday that investigators are aware of the threat made against federal buildings and are trying to determine whether the threat, which was reported Thursday, was credible.

“As always, we encourage people to remain vigilant and to promptly report suspicious activities which could represent a threat to public safety,” FBI spokeswoman Beth Anne Steele said in the statement.

The Portland office of the U.S. Bankruptcy Court was closed because of a threat of violence in the area, according to the court’s website. Also closed was the Mark O. Hatfield Federal Courthouse, which was the site of weeks of violent protests last month.

Two law enforcement officers, speaking to The Associated Press on condition of anonymity because they weren't authorized to publicly discuss the matter, said the threat warned of the intent to use a car bomb to target federal property. It wasn't clear if the threat was related to the protests.

Violent demonstrations have happened in Oregon's largest city for more than two months following the death in police custody of George Floyd in Minneapolis.

On Thursday police released information that showed during more than 80 nights of protests in Portland authorities declared riots more than 17 times and arrested more than 500 people.

The riot declarations allow police to use tear gas, flash bang grenades and other non-lethal weapons to try to break up crowds.

Portland police define riots as events "when six or more persons engage in tumultuous and violent conduct and thereby intentionally or recklessly creating a grave risk of causing public alarm, excluding persons who are engaged in passive resistance," The Oregonian/OregonLive reported.