The number of homicides reported in the United States rose last year, the FBI said on Wednesday, but the bureau warned that changes to its data collection program prevented nearly half of law enforcement agencies, including police in New York and Los Angeles, from requesting their data.

Overall, the FBI estimates that violent crime in the U.S. will drop by 1% in 2021. The robbery rate fell 8.9% from 2020 to 2021, which FBI officials said was largely driven by a decrease in overall violent crime, despite rising homicides and rapes nationally.

The number of homicides reported in the US has increased by 4.3% since 2020, according to FBI data. But top officials don’t believe that’s an accurate picture, as only 11,794 of 18,806 law enforcement agencies chose to provide expanded homicide data. They concluded that participation in 2021 remained “below a statistically acceptable level to be nationally representative”.

The incomplete data comes as the FBI and the Justice Department’s Bureau of Justice Statistics transition to a more detailed and comprehensive crime reporting system known as the National Incident Reporting System.

The new data collection method offers a more complete picture of crime in the country, with additional information collected on victims, offenders and arrestees, including age, gender and race, as well as a description of the relationship between the victim and the offender. The updated collection system also breaks down crimes by state and reports on suspected drug or alcohol use by offenders. Officials say the data will also provide more detailed information on specific crimes such as human trafficking, bribery and counterfeiting.

“The national homicide rate is up much, much less than we saw in 2020. And the violent crime rate nationally has actually slowed,” said Ames Gravert, senior counsel at the Brennan Center for Justice. “Both of these come as good news compared to what we saw in 2020.”

The homicide rate rose 29.4% from 2019 to 2020, FBI data previously showed.

“The catch is that all the information provided by the FBI is based on estimates due to changes in the way the FBI collects data,” Ames added. These changes resulted in “less accurate” information.

National estimates in 2021 are based on data submitted by 52% of law enforcement agencies across the country. Approximately 48% of law enforcement agencies nationwide did not report any data in 2021, leaving significant gaps in coverage.

“We’re certainly not saying it’s a fluke,” a high-ranking official from the Bureau of Justice Statistics told reporters on Tuesday, noting that the figures are obtained by estimation methods. “We have full confidence in these estimates. It’s not that we’re not confident in what they’re producing. There’s just a certain level of uncertainty around the estimates.”

But officials acknowledge that 2021 marks a transition year as more law enforcement agencies work to develop the tools and staff needed to collect more comprehensive data. Justice Department officials said analysts have prepared state-level estimates for about 40 of the 50 states, representing 65% of the total US population.

“While the FBI has used assessment methods to address data gaps, it is fundamentally difficult to do so without so much data,” said Rachel Eisenberg, senior director of criminal justice reform. Center for American Progress, a left-leaning think tank.

The FBI’s analysis of trends over the past year does not include most agencies in populous states like California, New York, Illinois, Pennsylvania and Florida. Major police departments failed to report data using the FBI’s new system, including the New York Police Department, Los Angeles Police Department, Los Angeles County Sheriff’s Office, San Francisco Police Department and Phoenix Police Department.

“The data is only as good as the input,” said Art Acevedo, a former Miami and Houston police chief. The former police chief praised the departments that participated in the FBI’s “more comprehensive reporting system.”

“Those who don’t… why aren’t they transparent about public safety?” Acevedo added. – What are they hiding?

The agencies had five years to switch to the new data collection system. But a senior FBI official said the transition was difficult for law enforcement for “a variety of reasons.”

“Whether it was funding or human resources. They may have had difficulty securing a supplier for their procurement cycles,” the official added.

“I can tell you that the departments that we are in constant contact with, they have full commitment to the transition,” the official continued. “They’re all working on it. It’s just that some did not come by the specified time.”