WASHINGTON — A new study of homegrown terrorism involving Muslim Americans suggests alarm bells set off by Congress over sleeper cells of Islamic extremists may be much too loud.

Study finds radical Muslim Americans are hardly a threat to United States.

Despite warnings of a potential wave of violent attacks hatched on U.S. soil, research by the Triangle Center on Terrorism and Homeland Security finds that the number of attacks committed by Muslim Americans has fallen for the second year in a row. According to the study, 20 Muslim Americans carried out or were arrested for violent terrorist crimes in 2011, down from 26 in 2010 and 49 in 2009.

Since the Sept. 11, 2001, attacks, the report said, 193 Muslim Americans have been arrested or convicted of violent terrorist acts. Last year was an average year for such offenses, the Durham, N.C.-based center said.

The findings, which follow revelations that the New York Police Department has spied on the city's Muslim community, are sure to bolster critics who warn that singling out Muslim Americans is counterproductive and bigoted, and may make the country less safe. 

"Muslim American terrorism continued to be a miniscule threat to public safety last year. None of America's 14,000 murders in 2011 were due to Islamic extremism," said Charles Kurzman, the University of North Carolina sociologist who wrote the study as well as the book "The Missing Martyrs: Why There Are So Few Muslim Terrorists." He said, "The challenge is for Americans to be vigilant about potential violence while keeping these threats in perspective."

David Schanzer, the Triangle Center's director, said the study proves that "those who predicted an inevitable, rapid increase of homegrown violent extremism among Muslim Americans were wrong." He added, "While homegrown radicalization is still a problem, the offenders from 2011 were less skilled and less connected with international 'terrorist' organizations than the offenders in the prior two years.”                        

— Huffington Post