Four people were killed and four wounded in a mass shooting at this Chicago house in June.

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CNN  — 

Mass shootings in the United States increased during the coronavirus pandemic, and even doubled in July 2020 compared to a year earlier, according to research published in the journal JAMA Open Network Thursday.

Researchers focused on data between April 2020 and July 2021 from the Gun Violence Archive on mass shootings, in which four or more people were killed or injured, not including the shooter.

In that 15-month period, there were 343 more mass shootings, 217 people killed and 1,498 people injured in the US, than expected.

The team observed an increase in mass shootings after May 2020, compared to trends in previous years. There were 88 such shootings in July 2020, 42 in July 2019 and 45 in July 2018, the team noted.

City police departments are also reporting an increase in gun violence during the pandemic. In Chicago, shooting incidents are up 64% so far this year compared with the same period two years ago.

Last year, there were 611 mass shootings around the country, compared with 417 a year earlier, according to Gun Violence Archives. This year there have been 498 mass shootings, 34 just in September so far.

After April 2020, the team said that there were an average of .78 additional daily mass shootings, .49 additional people killed each day and 3.40 additional people injured each day.

Increases in mass shootings during the pandemic were observed across the 882 cities included in the data, but cities with both low and high pre-pandemic mass shootings – as opposed to cities in the middle of the range – contributed most to the overall increase in fatalities.

The large increase in mass shootings during the pandemic is consistent with the idea that this violence may be influenced by social and economic factors, the researchers note.

CNN’s Theresa Waldrop contributed to this report.