WASHINGTON (AP) — Hate crimes in the U.S. remained relatively high last year after a surge not seen in nearly two decades, according to a new FBI report. But experts say it is actually an undercount because thousands of police departments, including some of the country's largest, didn't report their data.

Major cities like New York and Los Angeles, as well as large swaths of states including Florida and California, didn't send crime information to the FBI for 2021 due largely to changes in reporting requirements. The agency normally puts out the most comprehensive picture of hate crime in the nation, so this year's report is concerning for advocates trying to address spikes in hate crimes that have heightened fears among marginalized groups and sparked calls to address the issue head on.

"Hate crimes tear at the fabric of our society and traumatize entire communities," said Jonathan Greenblatt, CEO of the Anti-Defamation League, as he called on law enforcement to commit to hate-crime data collection. "The failure by major states and cities across the country to report hate crime data essentially — and inexcusably — erases the lived experience of marginalized communities across the country."

The troubling trend continued this year with a series of brutal, high-profile hate crimes, including a mass shooting at an LGBTQ nightclub in Colorado Springs and another that targeted Black people at a grocery store in Buffalo.

The nearly 7,300 hate crimes that were documented in the report are the third-highest total in the last decade and deeply alarming, Greenblatt said. The majority of victims were targeted due to their race, followed by sexual orientation and religion. But while the FBI report pointed to a small decrease from 2020 to 2021, other research has found that last year actually saw a troubling increase in hate crimes.

The Center for the Study of Hate & Extremism at California State University-San Bernardino found a 21% increase from 2020 to 2021 in an analysis of 20 states, drawing from police data.

In cities with more than 1 million people, meanwhile, hate crimes surged 39%, his research has found, with especially steep spikes in anti-Asian hate crime. Large cities are key to hate-crime data because they tend to be more diverse and have more ways for people to tell authorities about hate crimes, which are historically underreported, he said. But many were missing from this year's FBI report.

"We're talking about, quite possibly, a record for 2021 that America just doesn't know," said Brian Levin, a professor of criminal justice and the center's director. "All data has limitations, but this data is so incomplete as to leave out heavy swaths of places where the most terrorized communities live."

Less than two-thirds of the nearly 19,000 eligible law enforcement agencies reported hate crime data to the FBI for 2021, a steep decline compared to more than 80% the year before. That's because the federal agency switched to a new, more detailed reporting system. Police departments, including those in the nation's largest cities, said they weren't able to make the transition to the new system in time. Crime data collection from police departments has always been voluntary, and the bumpy transition also hampered violent crime data.

Federal authorities acknowledge the shortfall, but say the new system will eventually provide the country with a "richer and more complete picture of hate crimes nationwide," Associate Attorney General Vanita Gupta said in a statement. The Justice Department is doing a number of things to address the increase in hate crimes, including prioritizing those investigations at field offices around the country, awarding millions in grant money to local police and adding new victim resources in more languages, she said.

"The Justice Department is committed to prioritizing prevention, investigation and prosecution of hate crimes," she said. "The FBI's 2021 Hate Crimes Statistics are a reminder of the need to continue our vigorous efforts to address this pervasive issue in America."