The anonymous group mailed the list to several media outlets, law enforcement agencies and others this week, frightening the state's Hispanic community. A letter accompanying the list demanded that those on it be deported immediately.
The list also contains highly detailed personal information such as Social Security numbers, birth dates, workplaces, addresses and phone numbers. Names of children are included, along with due dates of pregnant women on the list.
Republican Gov. Gary Herbert wrote in a tweet Tuesday that he has asked state agencies to investigate the list's origin.
"We've got some people in our technology department looking at it right now," said Dave Lewis, communication for the state Department of Workforce Services. "It's a high priority. We want to figure out the how's and why's."
Lewis noted his department is one of several with access to the information included in the list. He said his agency didn't receive a copy of the list from the governor's office until late Tuesday.
Most of the names on the list are of Hispanic origin.
"My phone has been ringing nonstop since this morning with people finding out they're on the list," said Tony Yapias, former director of the Utah Office of Hispanic Affairs. "They're feeling terrorized. They're very scared."
The letter says some names on the list were sent to the U.S. Immigration and Customs Enforcement office in Salt Lake City in April. It says the new list includes new names, for a total of more than 1,300.
Included with the new letter is one dated April 4 addressed to "Customs and Immigration" and from "Concerned Citizens of the United States."
In the April letter, the writers say their group "observes these individuals in our neighborhoods, driving on our streets, working in our stores, attending our schools and entering our public welfare buildings."
"We then spend the time and effort needed to gather information along with legal Mexican nationals who infiltrate their social networks and help us obtain the necessary information we need to add them to our list," the letter says.
Agency spokeswoman Virginia Kice said ICE received a copy of the list, but she declined to say whether it is investigating the immigration status of the people on it.
"As a matter of policy, we don't confirm we are investigating an allegation or possible violation unless the inquiry results in some type of public enforcement action," Kice said.
She noted that because ICE has finite resources, it focuses its efforts "first on those dangerous convicted criminal aliens who present the greatest risk to the security of our communities, not sweeps or raids to target undocumented immigrants indiscriminately."
Kice added the agency has had a means for the public to report suspected criminal activity for several years - a 24-hour tip line staffed by trained enforcement personnel.