There may have been almost 100,000 more opioid-related overdoses between 1999 and 2016 than reported due to incomplete death records, a new study suggests.

Researchers at the University of Rochester found that 71.8% of unclassified drug overdoses during that period involved opioids, translating into 99,160 additional opioid-related deaths—28% more than reported.

The undercount was greatest in several states, including Alabama, Mississippi, Pennsylvania, Louisiana and Indiana, The Washington Post reports.

“A substantial share of fatal drug overdoses is missing information on specific drug involvement, leading to underreporting of opioid-related death rates and a misrepresentation of the extent of the opioid crisis,” lead researcher Elaine Hill, Ph.D. said in a news release. “The corrected estimates of opioid-related deaths in this study are not trivial and show that the human toll has been substantially higher than reported, by several thousand lives taken each year.”