Congressional investigators say the government is underestimating the threat of a chemical attack on America's densely populated cities and has failed to inspect virtually all of the chemical facilities that it considers particularly vulnerable to terrorists.

The investigation by Republican staff on the Senate Homeland Security Committee documents inspection delays and government errors in risk assessment in a $595 million terror prevention program. A copy of the report was obtained by The Associated Press.

Half of the 4,011 high-risk facilities on the Homeland Security Department's watch list are in 10 states. They are California, Texas, Ohio, Illinois, Pennsylvania, New York, North Carolina, Florida, Michigan and New Jersey.

Committee investigators say larger metropolitan regions such as Los Angeles, Chicago, New York and Philadelphia might be more vulnerable to an attack.