Hundreds of police departments in Texas and California failed to report officer-involved shooting deaths as required by law in the past decade, a recent study found.

Research by Texas State University in San Marcos found registries created by the two states to report all in-custody deaths did not list about 220 use-of-force fatalities in Texas and 440 in California from 2005-2015, the Houston Chronicle reported Monday.

The Texas and California attorney general’s offices and police officials in both states confirmed to the newspaper that many cases were missing.

“We’re not really blaming anyone - this is an incredibly complex problem,” said Howard Williams, one of the Texas State University professors who conducted the study. But he said it’s hard to change policy, improve training or purchase new equipment “when you simply lack the data to even know what’s going on.”

Texas and California are the only states to require reporting of all in-custody deaths, including jail deaths and officer-involved shootings.