There is no standardised system for recording shooting deaths across the country's thousands of law enforcement agencies
Data collected by the Washington Post newspaper suggests that the number of people shot by US police is twice as high as official figures claim.
The paper said that during the first five months of this year, 385 people - more than two a day - were killed.
The number of black people was disproportionately high among the victims, especially unarmed ones.
Official statistics rely on self-reported figures from law enforcement agencies.
They suggest about 400 people have been killed each year since 2008.
The US has seen a number of controversial cases where unarmed black people have been killed by white police officers.
A series of police killings of unarmed black people has sparked major protests
Police are allowed to use deadly force when they fear for their lives or the lives of others, however there is currently no reliable way of tracking police shooting deaths.
Instead, the government relies on self-reported figures from the nation's 17,000 law enforcement agencies. The figures exclude killings deemed not to have been justified.
The Washington Post says it logged every fatal shooting in 2015 by police in the line of duty using interviews, police reports, local media reports and other sources.
It found a homicide rate of almost 2.6 per day so far this year - more than double the average 1.1 deaths per day reported in FBI records over the past decade.
"These shootings are grossly underreported," former police chief Jim Bueermann told the newspaper. "We are never going to reduce the number of police shootings if we don't begin to accurately track this information."
Among the report's other findings: