(CNN) -- The Justice Department targeted American Muslims in "abusive counterterrorism sting operations" as part of aggressive terror investigations, an international human rights group alleges.
Human Rights Watch claimed in a report on Monday that it examined dozens of cases from investigation through prosecution, and found many defendants would never have committed a crime "if not for law enforcement encouraging, pressuring, and sometimes paying them to commit terrorist acts."
It alleges the FBI targeted people who did not appear to have any involvement in terror plots or related financing when the government began investigating them.
The FBI went after particularly vulnerable people, including those with intellectual and mental disabilities and the indigent, according to Andrea Prasow, deputy Washington director of Human Rights Watch and co-author of the report.
The Justice Department said the group's conclusions weren't accurate.
The federal government has prosecuted more than 500 terrorism-related cases since the September, 11, 2001, al Qaeda hijack attacks on the United States.
The report was based on more than 215 interviews with defendants, family members, attorneys, judges, current and former prosecutors and other experts. It specifically cited 27 cases.
In one example, a Boston-area man, Rezwan Ferdaus, pleaded guilty in 2012 and was sentenced to 17 years in prison for plotting to attack the Pentagon and the U.S. Capitol with model planes packed with explosives.
Human Rights Watch claimed the FBI targeted Ferdaus "even though his mental health was deteriorating significantly during the time" he was under investigation. The report said his father had to quit work to look after him.
"That didn't stop the FBI from pursuing a case by sending an informant in to work with Ferdaus and eventually help him develop a plot to attack" the Capitol, the group said.
The Justice Department said it has followed the law.
"The Department of Justice has been a steadfast ally of our nation's civil rights groups for decades. The report itself acknowledges that many of the decisions that the report criticizes are not only lawful but also specifically approved by federal judges, just as the material support statute that the report criticizes was upheld as constitutional by the U.S. Supreme Court. We do not and cannot target individuals solely for engaging in activities protected by the First Amendment, which includes free speech and religion," said spokesman Marc Raimondi.
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