(CNN) — One phone call saved countless lives and prevented a potential mass shooting during July 4 celebrations in Richmond, Virginia, the police chief and mayor said Wednesday.

The “hero” tipster overheard a conversation that a mass shooting was being planned for the celebration at the Dogwood Dell amphitheater and called that information into police on July 1, said Richmond Police Chief Gerald Smith during a news conference. The amphitheater seats 2,400 people, according to the city’s website.

The same day as the tip, police went to a residence at the 1000 block of Columbia Ave. where they found the weapons and ammunition. In total, police found two assault rifles, one handgun and 223 rounds of ammunition at the home, Smith said.

One man was arrested Friday and police put surveillance on a second person, who was arrested Tuesday, the chief said. Both suspects have been charged with being non-citizens in possession of a firearm.

“They were planning to, actually, shoot up our Fourth of July celebration,” Smith said. “We know what their intent is but we don’t have their motive, we do know that they lived at the same location.”

Both men are being held without bond at the Richmond City Jail, Smith said.

“Our officers quietly investigated and collaborated with their law enforcement partners and the community to stop what could’ve been a terrible day for the city of Richmond,” Mayor Levar Stoney said. “As we continue to live through an era, an age, of mass shootings and all around our country, no community is immune.”

The US Department of Homeland Security and the FBI were also involved in the investigation, Smith said. Police did not elaborate further on plans for the mass shooting but reiterated the importance of the tip.

“There’s no telling how many lives this hero citizen saved from one phone call,” Smith said. “Public safety is a responsibility of us all. One phone call saved numerous lives on the Fourth of July.”

The news out of Richmond comes just two days after a mass shooting left seven dead and dozens injured during an Independence Day parade in Highland Park, Illinois.

The carnage in Highland Park punctuates an already bloody American spring and summer — during the past 186 days, more than 300 mass shootings have happened in the US, according to data compiled by the Gun Violence Archive, a non-profit tracking such incidents.

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