WASHINGTON — A feared wave of homegrown terrorism by radicalized Muslim Americans has not materialized, with plots and arrests dropping sharply over the two years since an unusual peak in 2009, according to a new study by a North Carolina research group.

The study, to be released on Wednesday, found that 20 Muslim Americans were charged in violent plots or attacks in 2011, down from 26 in 2010 and a spike of 47 in 2009.

Charles Kurzman, the author of the report for the Triangle Center on Terrorism and Homeland Security, called terrorism by Muslim Americans “a minuscule threat to public safety.” Of about 14,000 murders in the United States last year, not a single one resulted from Islamic extremism, said Mr. Kurzman, a professor of sociology at the University of North Carolina.

The report also found that no single ethnic group predominated among Muslims charged in terrorism cases last year — six were of Arab ancestry, five were white, three were African-American and two were Iranian, Mr. Kurzman said. That pattern of ethnic diversity has held for those arrested since Sept. 11, 2001, he said.