Gun deaths reached the highest level ever recorded in the United States in 2020, the first year of the pandemic, the Centers for Disease Control and Prevention reported on Tuesday. Gun-related homicides in particular rose by 35 percent, a surge that exacted an unprecedented toll on Black men, agency researchers said.

“This is a historic increase, with the rate having reached the highest level in over 25 years,” Dr. Debra E. Houry, acting principal deputy director of the C.D.C. and the director of the National Center for Injury Prevention and Control, said at a news briefing on Tuesday.

More than 45,000 Americans died in gun-related incidents as the pandemic spread in the United States, the highest number on record, federal data show. But more than half of gun deaths were suicides, and that number did not substantially increase from 2019 to 2020.

The overall rise in gun deaths was 15 percent in 2020, lower than the percentage increase in gun homicides, the C.D.C. said.

The rise in gun murders was the largest one-year increase seen in modern history, according to Ari Davis, a policy adviser at the Johns Hopkins Center for Gun Violence Solutions, which recently released its own analysis of C.D.C. data.

He said preliminary figures suggest that gun deaths remained persistently high in 2021.

Federal officials and outside experts are not certain what caused the surge in gun deaths. The rise corresponded to accelerated sales of firearms as the pandemic spread and lockdowns became the norm, the C.D.C. noted.

But federal researchers also cited increased social, economic and psychological stressors; disruptions in routine health care; tension between police and community members following George Floyd’s murder; a rise in domestic violence; inequitable access to health care; and longstanding systemic racism that contributes to poor housing conditions, limited educational opportunities and high poverty rates.

Murders involving firearms were generally highest, and showed the largest increases, in impoverished communities.

“One possible explanation is stressors associated with the Covid pandemic that could have played a role, including changes and disruption to services and education, social isolation, housing instability and difficulty covering daily expenses,” said Thomas R. Simon, associate director for science at the agency’s division of violence prevention.

Black Americans remained disproportionately affected by gun violence in 2020. Firearm homicide rates increased by 39.5 percent among Black people from 2019 to 2020, to 11,904. The victims were overwhelmingly young men.

The Johns Hopkins analysis found that Black men aged 15 to 34 accounted for 38 percent of all gun murder victims in 2020, though they represented just 2 percent of the U.S. population.

Black men aged 15 to 34 were more than 20 times more likely to be murdered with a gun than white men of the same age. The number of Black women killed by guns also increased by almost 50 percent in 2020, compared to 2019, Mr. Davis said.

Rising rates of gun-related homicides were seen in all racial and ethnic groups, the C.D.C. said — except among Americans of Asian and Pacific Islander descent, who saw a small decrease.

Gun-related suicides have long been more common among older white men. But in 2020, rates rose mostly sharply among Native Americans and Alaska Native groups.

“Suicides impact a different population, typically middle-aged to older white men in rural communities,” Mr. Davis said. “We’re going to need to develop different types of solutions to deal with different types of gun violence.”