In a letter to Gov. Gavin Newsom, the task force, led by district attorneys from San Diego to Fresno Counties, asked for “significant resources” to combat “what appears to be the most significant fraud on taxpayer funds in California history,” and wrote that claims had been paid under the names of tens of thousands of incarcerated Californians.

In most cases, task force members said, the payments were sent in the form of prepaid debit cards to addresses designated on the applications, with the proceeds later deposited to inmate accounts in jail and prisons, but in some, the benefits were sent directly to the institutions.

Among the named beneficiaries: Cary Stayner, a serial killer who murdered four women near Yosemite National Park in 1999; Wayne Ford, another serial killer, who confessed to at least four murders in 1997 and 1998 in Northern California; Scott Peterson, convicted in 2004 of the murder of his pregnant wife, Laci; and Isauro Aguirre, who, with his girlfriend, tortured and murdered her 8-year-old son, Gabriel Fernandez, in 2013 in Palmdale, Calif.

“It’s behemoth,” said the Sacramento County district attorney, Anne Marie Schubert, who since last month has been chairing the task force. “And it’s not just happening here.”

In a statement, Mr. Newsom thanked the district attorneys “for their commitment to resolving this issue,” and said his Office of Emergency Services was creating a task force to coordinate state efforts.

“Unemployment fraud across local jails and state and federal prisons is absolutely unacceptable,” he said.

California, like many states, has struggled with unemployment abuse during the pandemic. When businesses began to shut down in the spring, a tidal wave of applications for benefits overwhelmed state unemployment systems and caused weekslong processing backlogs. Those delays worsened when Congress created new programs in response to the pandemic that made benefits more generous and expanded eligibility.

With millions of Americans out of work, states faced enormous pressure to start paying out benefits quickly. They were largely successful: By early May, some 30 million people were receiving payments, according to federal data. Economists widely credit the expanded benefits programs, along with other forms of federal aid, with stabilizing the economy and protecting millions of families from lasting financial harm.

But the rush to process unemployment applications also opened the door for fraud. The S.O.S. sent up on Tuesday in California by the district attorneys underscored the breathtaking scope of the state’s problem. And it drew attention to similar reports of fraud in Massachusetts, Illinois, Kansas and other states.

In August, a fraud ring operating out of the San Mateo County jail was broken up after law enforcement officers overheard inmates talking about fake claims being filed on behalf of inmates. The San Mateo district attorney, Stephen Wagstaffe, said 22 people have been charged so far. Most, he said, were already in custody but at least one remains at large on $1 million bail, at least some of which is believed to have been covered by the proceeds of unemployment fraud.

In September, the Beverly Hills police arrested 44 people and seized pre-loaded debit cards worth more than $2.5 million in unemployment benefits issued by the California Employment Development Department, which processes unemployment claims.

That month, a “strike team” formed by the governor to speed claims and address other chronic issues at the department issued a report recommending improvements, and the department suspended processing for two weeks to shore up controls and reduce a growing backlog. Mr. Newsom said in his statement that he had also instructed the department to address reports of fraud in correctional facilities.

But with only 17 fraud investigators at the department, the fraud reports persisted.

In October, federal authorities in Los Angeles arrested Fontrell Antonio Baines, a rapper who performs as Nuke Bizzle, for allegedly using identity theft to fraudulently obtain more than $1.2 million in federal pandemic unemployment assistance, which expanded existing benefits during the pandemic to self-employed and independent workers. In a YouTube music video, the musician held up stacks of pre-loaded debit cards issued by the Employment Development Department and another man rapped, “You gotta sell cocaine, I just file a claim.”

The district attorneys said they partnered with the California Department of Corrections and Rehabilitation to further investigate unemployment after the “strike team” failed to uncover the large kinds of unemployment fraud they were seeing. Mr. Newsom said the effort was also based on his direction.

Part of the problem, the task force wrote, was that, unlike at least 35 other states, California lacks the technology to crosscheck its state prison rolls and jail rosters against its lists of unemployment insurance claimants.

About two weeks ago, however, the group received a crosscheck from the U.S. Department of Labor between federal unemployment insurance claims data and a list of California prison inmates that federal officials had subpoenaed from the state.

The 35,000-plus payments sent out in the name of state prisoners alone, they discovered, totaled more than $140 million between March and August, with nearly a half-million dollars disbursed in the names of 133 death row inmates. Among the largest: a claim for $19,676 disbursed in the name of a death row inmate and another for $48,600 in the name of another felon.

The crosscheck did not include the state’s 58 county jails, state hospitals where sexually violent predators and mentally ill inmates are incarcerated or other institutions where people are being held under civil commitments.

However, Ms. Schubert said, the task force investigation has uncovered fraudulent claims filed under the names and Social Security numbers of inmates at every level of the corrections system, including most of the jails, all of the state’s 30-plus prisons and thousands of federal prisoners.

A spokeswoman for the Employment Development Department said it would be “pursuing how to integrate such cross-matches moving forward as part of enhanced prevention efforts during this unprecedented time of pandemic-related unemployment fraud across the country. In addition, EDD is working collaboratively with state cybersecurity experts.”

“The volume of fraud as well as the types of inmates involved is staggering,” the task force wrote to Mr. Newsom, adding that many of the claims filed under the names of inmates were being paid to recipients in other states and, in some cases, other countries.

“This needs to be halted,” Ms. Schubert said. “We are paying hundreds of millions of dollars in the name of serial killers, rapists and child molesters. We need to turn the spigot off.”

Ben Casselman contributed reporting from New York.