An Associated Press investigation finds that more than 2,700 city police and county sheriff's departments have not filed a single hate crime report with the FBI over the last six years. And thousands more had gaps in reporting.

Each year, the FBI issues a report on the number of hate crimes in the U.S. It's based on reports submitted voluntary by law enforcement agencies.

Some of the non-reporting agencies say they thought they were reporting, while others thought they didn't have to file.

Activists worry the lack of a comprehensive count masks the true scope of hate in the U.S. at a time of heightened racial, religious and ethnic tensions.

The FBI has called on all law enforcement agencies to be more aggressive in tracking and reporting hate crimes.