The estimated number of violent crimes in the U.S. decreased slightly in 2021, according to statistics released by the FBI — but the data is incomplete.

Why it matters: The figures from President Biden's first year come as Republicans seek to make crime a major issue before the midterms, an issue that polls show voters give an edge to the GOP.

By the numbers: Violent crime volume decreased by 1% in 2021, from 1,326,600 in 2020 to 1,313,200.

• The estimated number of murders increased from 22,000 in 2020 to 22,900 in 2021, a 4.3% increase.
• Rates of robberies decreased 8.9% from 2020 to 2021, contributing significantly to the decrease in overall violent crime.

Yes, but: Nearly 40% of law enforcement agencies nationwide, including the New York City Police Department and Los Angeles Police Department, failed to report their 2021 crime data to the FBI, according to data provided to Axios Local from a partnership with The Marshall Project.

• That will result in a data gap that experts say makes it harder to analyze crime trends and fact check claims politicians make about crime, reports The Marshall Project's Weihua Li.
• The FBI's annual data set is the country's foremost way to understand how crime across the U.S. is changing, measuring things like how many murders or rapes took place last year or how many people were arrested.

Between the lines: Social-economic changes, like the pandemic, inflation and economic uncertainty have contributed to jumps in crime, Robert Arcos, chief of the Bureau of Investigation in the Office of the Los Angeles County District Attorney, told Axios.

Meanwhile, police departments are experiencing officer shortages and pressures to reform policing tactics two years following the murder of George Floyd.

Flashback: The estimated number of violent crimes across the U.S. rose for the first time in four years in 2020, according to statistics released by the FBI last year.

• The estimated rate of violent crime — such as murder, rape, robbery and aggravated assault — was 387.8 offenses per 100,000 inhabitants, a 5.6% increase from 2019's rate.

Yes, but: Violent crime rates in 11 of the largest communities along the U.S.-Mexico border stayed below the national average in 2020, an Axios analysis of that year found. 

• Roughly half of America's medium- and smaller-sized communities with majority Black populations actually saw drops in overall reported violent crime in 2020, per Axios analysis.

The bottom line: Trends in crime are connected to nuance and complexity that are difficult to expand in soundbites but easy to exploit.

• Fears about crime historically have helped Republicans who take "tough on crime" approaches that poll well.

Editor's note: This story and headline were updated to reflect that the FBI data is incomplete.