By Brendan O'Brien

(Reuters) -A shooting at a holiday block party in Baltimore over the weekend that left two people dead and 28 injured highlights a sustained surge in mass shootings in the United States since the beginning of the COVID-19 pandemic in 2020.

As of Monday, 340 mass shootings have taken place across the country so far this year. At that pace, the United States will experience 679 mass shootings in 2023, the second most in a calendar year since 2014, according to the Gun Violence Archive.

In 2019, the year before the pandemic caused far-reaching changes in the way Americans work, play and interact with one another, the number of mass shootings reached 417, or about 61% fewer than the projected total for 2023.

The rise in mass shootings over the last three years is partly due to the impact the pandemic has had on Americans, according to James Alan Fox, a criminologist at Northeastern University.

"Lots of people are struggling, financially and emotionally, from the pandemic," Fox said, adding that gun sales have increased since the beginning of COVID-19. "Then you also have the tremendous divisions now and polarization in our country."

Gun ownership, guaranteed under the second amendment of the Constitution, is widespread in the United States, and it is a contentious political issue that heats up after particularly shocking shootings such the one that took place in Baltimore.

"We're still dealing with mass shootings because of the inaction to deal with this issue on a national level," Baltimore Mayor Brandon Scott said on Monday during an interview on CNN. "This can no longer be an issue that falls at the feet of local police, local elected officials or state governments."

On Sunday, gunfire erupted at a outdoor neighborhood block party in South Baltimore's Brooklyn Homes community. Among the two people dead and 28 others injured, about half were children. Investigators have asked for the public's help in tracking down multiple suspects.

At the pace of the first half of this year, the number of mass shootings over the 2023 calendar year would reach 679 or about double the 336 recorded in 2018. It would mark the second highest annual total over the last nine years, behind only the 690 recorded in 2021, according to data collected by the nonprofit group that tracks shootings.

The past weekend alone saw five other mass shootings, including one that wounded eight at a Wichita, Kansas, nightclub early on Sunday, according to the Gun Violence Archive, which defines a mass shooting as any in which four or more people are wounded or killed, not including the shooter.

Among the deadliest mass shootings in 2023 was an attack in a Monterey Park, California, ballroom that left 11 people dead on Jan. 21 and another in Allen, Texas, where eight people were killed at a mall on May 6.

The definition of a mass shooting differs among organizations and media outlets that track gun violence.

For example, the online database Mass Shooting Tracker defines a mass shooting as any in which four or more are shot, including the shooter or shooters. The FBI defines a mass shooting as one in which at least four people are murdered with a gun.

(Reporting by Brendan O'Brien in Chicago; editing by Grant McCool)