Meanwhile, authorities say they arrested a "person of interest" in connection with the shooting that left three men dead at a busy tavern in southeastern Wisconsin on Sunday.
The rampages are just the latest of several shooting attacks across the nation, including one at a FedEx warehouse in Indianapolis last week that left eight people dead.
Mass killings have claimed four or more American lives every week for the past six weeks, leaving dozens dead and several more people wounded.
The Gun Violence Archive, which defines a mass shooting as when four or more people have been shot or killed, not including the shooter, says there have been a total of 151 mass shootings in the USA this year. This means the nation has averaged more than one mass shooting a day since the year began.
In 2019, there were 417 mass shootings, up from 337 in 2018.
The archive says that this year, the US is on pace to have 511 mass shootings — more than any year between 2014 and 2019 — though mass shootings are historically more likely to occur during the latter half of the year, meaning the rate could increase.
It says the total number of Americans killed by gun violence since the start of the year is 12,654, with 89 of them being children under the age of 11, and 321 of them being teenagers aged 12-17.
In a series of maps that show just how disturbing an issue gun violence has become, the archive has pinpointed every gun death, every death of a child or teenager from gun violence and every mass shooting.
Meanwhile, leading US health expert Dr Anthony Fauci has weighed in on gun control as he branded the recent spate of mass shootings across the country a "public health emergency".
"When you see people getting killed, I mean, in this last month, it's just been horrifying what's happened," the president's chief medical adviser told CNN on Sunday. "How can you say that's not a public health issue?"
Last month, the US House of Representatives passed gun control legislation that would expand background checks on all commercial firearm sales and transfers, including those sold between parties, online, and at gun shows.
The bill needs to pass the Senate for the legislation to be approved, but Republican members are resisting the outlined gun reform, according to CNN.
Here are just some of the mass shootings that have rocked the US this year:
At least eight people were killed, not including a gunman who was believed to have taken his own life after opening fire in a FedEx warehouse. Family members said that workers were unable to use their mobile phones on the job, leaving them unable to confirm their safety for hours.
April 8: Texas
One person was killed and five others were wounded during a workplace shooting at a manufacturing facility, Kent Moore Cabinet, in Bryan, Texas. The suspected gunman, Larry Bollin, 27, an employee, was arrested in a nearby county, and one state trooper was also shot and wounded at that time.
March 31: California
A gunman opened fire at a real estate office, killing four people, including a 9-year-old boy. The shooting was most likely related to a "business and personal relationship which existed between the suspect and all of the victims", authorities said.
In late March, a gunman killed 10 people, including a police officer, after walking into a Boulder, Colorado, grocery store and randomly shooting at shoppers. Police say suspect Ahmad Al Aliwi Alissa, 21, bought a high-powered weapon less than a week before the shooting.
March 16: Georgia
Eight people, including six women of Asian descent, were killed at three spas, at least two of which had been frequented by the gunman. The attack caused particular alarm among many Asian-Americans.
February 2: Oklahoma
Five children and a man were killed and a woman was seriously injured when they were shot at a home. A brother of one of the victims was arrested at the scene. The police said they believed the victims were related.
Five people, including a pregnant woman, were found dead inside a home after the authorities came into contact with a juvenile male with gunshot wounds. A day after the shooting, he was arrested.
January 9: Illinois
Five people were killed, including a 15-year-old, and two injured in a "murderous rampage" in Chicago and nearby Evanston. The suspect, Jason Nightengale, 32, was killed after a shootout with police.