Roommates were scheming to 'shoot up' Fourth of July celebration in Virginia, authorities said. Police thwarted the planned attack thanks to a 'hero citizen' who overheard the plot.

• Virginia police thwarted a potential shooting at a Fourth of July celebration, officials said.
• The possible attack was prevented thanks to a "hero citizen" who called in a tip, police said.

A planned mass shooting attack at a Fourth of July celebration in Richmond, Virginia, was thwarted by police, thanks to a "hero citizen" who overheard details about the plot and alerted authorities, officials said on Wednesday.

"There is no telling how many lives this hero citizen saved from one phone call," Richmond Police Chief Gerald Smith told reporters during a press conference.

The tip, which cops received on July 1, prompted police to launch an investigation in collaboration with the US Department of Homeland Security and the FBI, and it led authorities to the home of the suspects, roommates Julio Alvarado-Dubon, 52, and Rolman Balacarcel, 38.

Inside the residence, police discovered "in plain view" two assault rifles, a handgun and 223 rounds of ammunition, according to Smith.

Alvarado-Dubon was swiftly arrested and charged with being a non-US citizen in possession of a firearm, while Balacarcel was arrested days later on July 5 on the same charge after cops surveilled him.

"We did not have probable cause to arrest him at that time, so we kept him under surveillance," Smith said.

It was not immediately clear whether the suspects had retained attorneys.

"They were planning to shoot up our Fourth of July celebration" at Richmond's Dogwood Dell Amphitheater, Smith said.

A motive was not immediately known.

Smith hailed the local man who "overheard" a conversation that a "mass shooting" was being planned for Richmond's Independence Day celebration and tipped off police.

"One phone call saved numerous lives on the Fourth of July," Smith said, adding, "We are so thankful for him."

Richmond Mayor Levar Stoney also praised the tipster and the police for preventing "what could have been a terrible day" for the city.

"It's very, very important that we remember the phrase 'see something, say something,'" Stoney said, explaining, "that phrase came in handy this past weekend."

The news of the thwarted potential attack comes on the heels of the Fourth of July parade mass shooting in Highland Park, Illinois, which left seven dead and dozens injured.

"I am just outraged," Smith said, as he commented on the recent mass shootings that have plagued the nation.

"It's ridiculous," the police chief said. "At some point in time this has got to stop."