SAN JOSE — As the novel coronavirus spread through the Bay Area and across California in March, deaths in Santa Clara County rose significantly compared to the same time period a year before, adding to concerns that the toll of the virus may be more widespread here than initially believed.

New data released to this news organization shows that deaths recorded by the Santa Clara County Medical Examiner-Coroner’s Office rose 20% last month, compared with March of 2019 — an increase that includes a 17% rise in the number of people who died at home. Overall, COVID-19 was listed as the cause of death or a significant condition for 32 people who died in the county in March — about half of the overall increase, though county officials acknowledge many more infections likely went undiagnosed.

The Santa Clara data is the most detailed death information released so far by any county in the Bay Area. The emerging pattern mirrors what has been seen in some other places in the country, especially in New York City, which last week boosted its coronavirus death totals substantially based on new estimates of undiagnosed cases.

As its investigators zero in on the most suspicious cases, the Santa Clara County public health department revealed Tuesday that its first recorded coronavirus death occurred on Feb. 6 — three weeks earlier than what was previously thought to be the first U.S. death, in Washington state on Feb. 28. The county also disclosed two additional deaths from coronavirus, on Feb. 17 and March 6, which predate the county’s first publicly reported death on March 9. All three individuals died at home and were tested posthumously for the virus.

The figures released by the medical examiner — which include deaths related to COVID-19 and also from other leading causes such as accidents, heart disease, cancer and diabetes — are just a snapshot of the overall deaths that have occurred in the county amid the pandemic.

County Executive Jeff Smith said the uptick in deaths may be even higher — up to 25% compared to March 2019, with a rise in deaths at home between 20% and 21% — and include more COVID-19 deaths than previously known. He said COVID-19 fatalities appear to have accounted for about 41% of the increase in the total number of deaths.

“We know the virus has dramatic effects. It’s not the kind of virus that waits around — some people are dead within a few days of diagnosis,” wherever that happens, Smith said. “It’s a much more dangerous virus than we initially recognized.”

Experts said the county death tolls deserve more study but suggest the coronavirus death toll is understated, either because not enough people have been tested or fear of the virus is scaring sick people away from hospitals.

“April could turn out to be different or February could have been very different. It’s important to raise the question,” said John Swartzberg, clinical professor emeritus of infectious diseases and vaccinology at UC Berkeley. “It will be interesting to see if it turns out to be the case.”

As of Tuesday, Santa Clara County had reported 1,948 confirmed coronavirus cases and 88 coronavirus-related fatalities. The death toll is the highest of any county in the Bay Area, accounting for about 40% of total deaths in the region.

Santa Clara County Supervisor Dave Cortese, who has advocated for more detailed data to be released about COVID-19 infections and deaths, said he has been disappointed with what he described as “sanitized reports” that offer a limited picture of the virus’s spread in the county.

“At some point you put two and two together. If we don’t have adequate testing by public health’s own admission, or adequate screening, then how can these death numbers be possibly accurate?” he said.

He added, “Everybody knows we are doing one of the best jobs in the country of responding to it, so why bury the lead? It’s important to know that somebody was infected early, especially in the global context,” Cortese said. “But there’s a whole other story: By the way, our deaths are 25% higher than last year. When were you going tell somebody that?”

New York City last week dramatically increased its COVID-19 death toll by adding more than 3,700 victims presumed to have died as a result of the virus but never tested positive. The revised death tally brought the number of people killed in the city to more than 10,000 and increased the national count by 17%.

Though California has yet to take any similar steps to adjust how coronavirus deaths are counted, Gov. Gavin Newsom said Wednesday the state has directed coroners to revisit deaths dating back to December “to ultimately help guide a deeper understanding of when this pandemic really started to impact Californians directly.”

“When this occurred is important forensic information that is going to be profoundly significant in terms of understanding the epidemiology of the disease,” Newsom said.