The number of deaths due to opioid overdoses may be much higher than previously thought, according to a new study.

Health officials have said opioids have caused at least 400,000 deaths in the United States in the past two decades, Scientific American reports. The new study analyzed data from 630,000 people who died of drug overdoses between 1999 and 2016. They separated the deaths into those linked to a specific drug, and those not linked to any drug.

They found that 71.8% of unclassified drug overdoses involved opioids, translating into 99,160 additional opioid-related deaths—approximately 28% more than previously reported.

The researchers found a striking geographic variation in undercounting of opioid overdoses. In Alabama, Indiana, Louisiana, Mississippi and Pennsylvania, the number of apparent opioid-related deaths more than doubled under the new analysis.

“Opioid deaths serve as one of the main measures of the opioid crisis, and if opioid deaths are not counted accurately, the extent of the crisis can be severely misrepresented,” said study co-author Elaine L. Hill of the University of Rochester Medical Center.