Americans who land on ISIS "hit lists" may never find out they have been marked for death by the terror group, said a key lawmaker Tuesday, as he called on the FBI to explain its obligations and procedures in contacting potential targets.

Several of the murky lists have appeared online, posted on various sites and social media platforms linked to ISIS. One list obtained by FoxNews.com Wednesday detailed 12,000 Canadian residents, along with their addresses and phone numbers. Even though the lists seem to be random and people named on them have not yet been targeted, law enforcement officials have notified many -- but not all -- of the individuals named.

“There shouldn’t be any question in the minds of FBI officials that Americans deserve to know if they are on an ISIS hit list,” Sen. Charles Grassley, R-Iowa, wrote in a Tuesday letter to FBI Director James Comey.

In April, FoxNews.com reported that thousands of New Yorkers were being informed by NYPD detectives that their names have appeared on a list of “potential targets.” One person contacted by the NYPD told FoxNews.com she had no idea why her name would be on an ISIS list, but said police told her to report anything suspicious, including any anomalies in her banking and credit transactions.

Grassley, citing a Monday report by Circa News, said other police departments around the nation did not take similar action. Circa contacted dozens of people whose names appeared on the list in New York, Texas, Florida and California and found the overwhelming majority had no idea they were on ISIS’ radar.

“Reportedly, the FBI even neglected to notify the relevant local police departments of these specific threats, which urged followers of the terrorist group to assassinate American citizens,” Grassley said. “These alleged failures are alarming, given the FBI’s stated commitment to ‘work closely with federal, state, and local partners to keep the public informed of potential threats.’”

The FBI would not comment on the report, but issued a general statement about how it "routinely notifies individuals and organizations of potential threat information."

The task of informing thousands of people their names appear on these lists is taxing on the agency’s resources, said John Cohen, the former counterterrorism coordinator at the Department of Homeland Security and currently a professor at Rutgers University in New Jersey. And the lists may not even be significant, he added.

“I can download a phone book and say, okay ISIS supporters, go kill these people,” he said.

Cohen said it is up to the agency to determine those on the list that appear most at risk, whether it be public profile or specifics of the threat.

In October 2014, Fox News reported that U.S. military personnel were sent an Army intelligence bulletin warning them to be vigilant after Islamic State militants called on supporters to scour social media for addresses of their family and to “show up and slaughter them.”

Canadian officials had no immediate comment on the latest list, but in June phoned about 151 citizens who appeared on kill lists, according to CBC.ca.

“When this kind of a threat to Canadians is put into the public domain, all of the security, and police, and intelligence agencies of the Government of Canada respond in a robust way to keep the country safe,” Public Safety Minister Ralph Goodale said.