An 18-year-old man executed a Native American couple who stopped to help him on a Montana roadside Wednesday because their daughter "laughed" at him, an FBI agent said in court Thursday.

Jason Shane, 51, and Tana Shane, 47, died Wednesday following the shooting on an Indian reservation in the small town of Pryor, and their daughter, 26-year-old Jorah Shane, was shot in the back when she tried to run away, the woman's aunt, Ada Shane, told The Associated Press. Jesus Deniz, also known as Jesus Deniz Mendoza, of Worland, Wyo., told federal investigators he shot the three, who had stopped to help him because Jorah Shane laughed at him.

“Deniz told the interviewing agents that he shot the victims because he was getting tired of waiting around, and because the daughter laughed at him,” a court document submitted by federal authorities said. The FBI and Bureau of Indian Affairs are handling the investigation because the murders occurred on an Indian reservation and the victims were members of the Crow tribe.

FBI agents say Deniz, a Mexican national and green card holder, “admitted to shooting three people with a .22-caliber rifle and then driving away from the scene in the victims’ vehicle.”

“Deniz told the interviewing agents that he shot the victims because he was getting tired of waiting around, and because the daughter laughed at him.”

- Court document submitted by FBI

The family came to help Deniz, who had run out of gas, after Tana Shane spotted him on her way home and went to her house just 50 yards away to get her husband and daughter, according to the Billings Gazette.

But when the trio approached the suspect, Deniz “pointed a gun at them, and told them to get out of their car,” and then demanded money, the complaint states. When the couple said they had only change because they recently returned from a religious revival in Window Rock, Ariz., Deniz told them to “start walking,” the complaint says. Jorah Shane told investigators she heard gunshots and her mother telling her in Crow to run.

As she ran, she “felt blood running down her face,” which was the result of a bullet wound, according to the document.

“She heard another gunshot and felt a bullet hit her in the back,” the complaint says. Jorah Shane then watched helplessly as the suspect drove away in the family’s car, but her calls for help were heard by several people at the nearby St. Charles Mission School.

Deniz, who was out on bond after being arrested last month for burglary in Wyoming, was nabbed 120 miles away near Meeteetse, Wyo., early Wednesday afternoon.  He was being held in Park County, Wyo., pending an appearance at 3 p.m. Friday in front of U.S. Magistrate Judge Carolyn Ostby in Billings.

A Department of Homeland Security official said Deniz was legally admitted into the U.S. on May 31, 2013. Immigration and Customs Enforcement officials were aware of the burglary charge, but cannot deport Deniz unless or until he is convicted of a crime.

“This individual does not have any criminal convictions, and, as a permanent resident, is not currently removable," ICE said in a statement. "Thus, an ICE detainer cannot be placed on the individual at this time. However, ICE is closely monitoring this case and coordinating with local authorities. If he is convicted for a criminal offense that allows him to be removed from the country, after the completion of sentence, ICE intends to take him into custody and pursue his removal from the United States."

Jorah Shane, who has four sisters and two brothers, did not know her parents were dead before she went into surgery Wednesday night for the bullet lodged in her spine, according to the Gazette.

"Last night before she went in, she told everyone to go look for her mom, she's hiding in the field," Ada Shane told the newspaper.

Ada Shane said it was just like her brother and his wife to help a stranger.

On Wednesday morning, Tana Shane drove by a young man who said he had run out of gas on the road less than 50 yards from her home, Ada Shane said.

"Both my brother and sister-in-law have big hearts," she said. "They're always helping someone else."

The Associated Press contributed to this report