(RICHMOND, Va.) -- News of the thwarted attack comes after seven people were killed and dozens more injured in a mass shooting at a July Fourth parade in a Chicago suburb on Monday.
"There is no telling how many lives this hero citizen saved from one phone call," Richmond Police Chief Gerald Smith told reporters Wednesday.
Richmond police received a tip from a citizen on July 1 who "overheard a conversation that there was a mass shooting being planned here," Smith said.
Acting on the tip that day, police began an investigation along with Homeland Security, Smith said. Officers responding to an apartment in Richmond "saw evidence in plain view that corroborated the hero witness' statement," Smith said.
Officers seized two assault rifles, one handgun and 223 rounds of ammunition, Smith said. The suspect, Julio Alvarado-Dubon, 52, was taken into custody and charged with possessing a firearm as a non-U.S. citizen.
Police surveilled Alvardo's roommate, identified as Rolman Balacarcel, 38, for several days before he was arrested on Tuesday in Albemarle County, Virginia, on the same charge. Additional charges could be possible for both suspects, Smith said.
Authorities allege the two were plotting a mass shooting at a Fourth of July celebration at the Dogwood Dell Amphitheater. The Diamond baseball stadium was another area of concern, Smith said.
"They were planning to actually shoot up our Fourth of July celebration," the chief said. "We know what their intent is, but we don't have their motive."
The two suspects were not previously known to Richmond police. They are being held in jail on no bond, Smith said. It is unclear if the suspects have an attorney.
Investigators are working to trace the weapons seized, Smith said. The FBI is also involved in the investigation, he said.
The chief did not provide any further details about the tip, though lauded the man who contacted the police.
"We owe several lives to that one person," Smith said.
News of the alleged mass shooting comes after the 21-year-old suspect in Monday's Highland Park shooting confessed to the massacre, prosecutors said Wednesday. The suspect allegedly contemplated another attack that day in Madison, Wisconsin, authorities said.
"The success of this particular investigation can only be juxtaposed against the horrors in which the rest of the country has seen," Smith said.
Richmond also experienced a mass shooting early on July Fourth at an after-hours club, where six people were wounded.
Richmond Mayor Levar Stoney called gun violence in the city and nationwide an "epidemic" and urged state and federal lawmakers to change U.S. gun policies.
"We need more -- more policies that will keep people safe, so that these firearms and weapons of war don't get into the hands of the wrong people," Stoney told reporters Wednesday.