The two Mexican citizens who murdered a coworker at a Michigan business last summer were in the country illegally and worked for a dairy farm that was a registered user of the federal government’s E-Verify system, aimed at preventing unauthorized immigrants from attaining work, according to a federal law enforcement official and government data.

Siblings Francisca Vargas-Castillo and Leobardo Torres-Castillo separately entered plea deals in Michigan’s Sanilac County Circuit Court earlier this month for the death of Bricia Flores-Rivera, who had worked with the two at a farm north of Detroit.

The deceased victim, Flores-Rivera, had also been illegally present. A DHS official said the department does not know when the three entered the U.S. and how long they had been in the country.

Flores-Rivera’s body was found by a local law enforcement officer in Buel Township, Mich., Sept. 1, 2018. Her body was retrieved from a culvert, a tunnel meant to carry water under a road or railroad.

A witness at the trial said the siblings believed Flores-Rivera was trying to poison Vargas-Castillo’s children and seduce her husband. Vargas-Castillo had asked her brother to help her confront the woman. The witness said she drove the siblings to a trailer in Marlette Township, Mich., and was told to park in a cornfield behind the trailer but observed Torres-Castillo beat the woman with a stick before Vargas-Castillo then began stabbing the victim.

[Opinion: Trump urged to nationalize ‘E-Verify’ after 700 percent surge in arrests of illegal workers]

Vargas-Castillo admitted to second-degree murder, tampering with evidence in a criminal manner, and concealing the death of an individual. Her brother, Torres-Castillo, confessed to charges of assault with intent to do greater bodily harm less than murder and the tampering and concealing charges. Both will be sentenced June 26.

The two assailants and one victim had worked for Goma Dairy Farm in Marlette, Mich. The company was founded in 1999 by Geert and Gertie van den Goor. The married couple moved from the Netherlands to the U.S. and were recognized in 2014 as the Michigan farm of the year.

The company is a registered user of E-Verify, an online tool that allows employers to input a new hire’s driver’s license and Social Security information in order to verify the person’s identity is legitimate and eligible for work.

E-Verify was rolled out as a pilot program by the Clinton administration in 1996 and is now administered by the Department of Homeland Security’s U.S. Citizenship and Immigration Services. Employers must register with USCIS to use the system. Employers can choose whether or not to sign up. Knowingly hiring an undocumented immigrant is a federal crime.

A list of E-Verify participants shows Goma Dairy Farm is a registered user of E-Verify, though it’s not clear how long it has used the system. The owners were not able to be reached for comment.

(Screenshot via E-Verify.gov)

A USCIS representative told the Washington Examiner that being a registrant does not necessarily mean a company used the system before hiring employees.

“The search tool only includes employers who are registered to use E-Verify, so all employers listed in the search tool are registered users of the service,” the official wrote in an email.

The DHS official said the two siblings may have used stolen documents, fake documents, or not actually been cleared through E-Verify — similar to what happened in the hiring of an unauthorized Mexican immigrant who is on trial in Iowa for the murder of a college student that also took place last summer.

The murder of Iowa college student Mollie Tibbetts became a national story after an unauthorized immigrant from Mexico was charged with her death. Tibbetts went missing in July and her body was found in late August. Tibbetts' alleged killer, Cristhian Rivera, was employed at the time by Yarrabee Farms in Brooklyn, Iowa.

Initially, the farm owner said he ran Rivera’s credentials through E-Verify at the time he was hired. However, farm co-owner and manager Dane Long later clarified that he ran the state-issued photo identification card and Social Security card that Rivera provided through a separate database, the Social Security Number Verification Service.

Rivera had used documents that belonged to a “John Budd” and was cleared by the SSN checker as being a legitimate identity eligible to work.

"We learned that our employee was not who he said he is,” said Lang.