Washington —- Tobacco companies have manipulated menthol levels to attract young cigarette smokers and keep older ones, researchers at the Harvard School of Public Health reported Wednesday.

Their finding, with which industry spokesmen disagree, is based on a review of more than 500 internal tobacco-industry documents dated from 1985 through 2007.

The documents showed, according to the researchers, that tobacco companies studied how controlling levels of menthol could increase brand sales. They concluded that new and young smokers liked mild menthol that masked the harshness of tobacco smoke. Veteran smokers, the companies are said to have concluded, favored stronger doses of menthol for its cooling effects on their throats.

The findings come as Congress weighs whether to grant the U.S. Food and Drug Administration authority to regulate tobacco products, including additives, at the national level. The bill would allow the FDA to ban all cigarette flavorings except menthol. If FDA tests of menthol showed that it added to the health risks of smoking, the agency could ban menthol, too.

No conclusive evidence shows menthol cigarettes to be more harmful than conventional ones, said Terry F. Pechacek, the associate director of the Office of Smoking and Health at the Centers for Disease Control and Prevention in Atlanta. Pechacek said, however, that there was some evidence that menthol smokers had a harder time quitting.

Menthol has proven appeal to young people. According to the 2006 National Survey on Drug Use and Health, 44 percent of smokers ages 12 to 17 reported that they smoked mentholated cigarettes. Menthol also is popular among African-American smokers, two-thirds or more of whom smoke mentholated brands, according to Gregory N. Connolly, a co-author of the report and the director of Harvard's Tobacco Control Research Program.

According to the program's lab tests of menthol concentrations in cigarettes since 2000, menthol went down in brands that the young preferred, such as Newport, Salem Black Label and Kool Milds.

It went up in brands such as Marlboro Menthol, which were aimed at older smokers.

Behind the moves, the researchers assert, was an effort to woo new smokers. They said the "rapid introduction" of new milder menthol brands in the past decade violates a provision in the Master Settlement Agreement of 1998 between tobacco companies and state governments that prohibits them from directly or indirectly targeting youths.

Michael Robinson, a spokesman for Lorillard Tobacco Co. in Greensboro, N.C., called the report's findings that menthol was manipulated to target young smokers "categorically false."

"Lorillard does not control levels of menthol to promote smoking among adolescents and young adults," he said in a statement.