A terrorist attack at any of some 2,500 U.S. chemical plants could harm at least 10,000 residents, says a new report by the Congressional Research Service (CRS). The government research organization’s report also found that an attack at 6,700 plants could endanger more than 1,000 people; the Department of Homeland Security recently said its own analysis found that only 3,400 plants held the risk of harming more than 1,000 people if attacked.

In its report, CRS compiled and updated risk management plan (RMP) data reported by companies to the Environmental Protection Agency, starting in the late 1990s. In recent years, a component of the RMP data, the worst-case scenarios, has been used to assess the potential impact of a terrorist attack on a chemical plant. CRS found that the universe of chemical plants reporting this data had dropped from 14,400 to 12,099 facilities and attributed this decline to company noncompliance, bankruptcy, or process changes eliminating use of toxic or flammable chemicals.

CRS also found that the number of plants that could threaten more than a million residents declined slightly from earlier EPA data but still include 106 facilities in 23 states; Texas, California, and Illinois have most of these plants. Texas and California also have the most facilities where more than 10,000 people would be affected by a severe accident or attack. Both have about 300 plants with this potential.

The report was requested from CRS by Rep. Edward J. Markey (D-Mass.), who has urged mandatory plant security requirements for these plants.