Laoshi Dominguez was working as a waitress in Texas. In the back he found the rifle under the table and pulled it over the shoulder so as not to get his hand dirty.
Mr Dominguez was shot dead in front of his wife and three children – all aged about two.
Officers came to the scene after the family phoned emergency services, who sent a vehicle to pick up Mr Dominguez’s corpse.
‘Covered in blood’
Image caption The teenager left a note behind in which he said he thought his victims were robbers
The body of his wife, his four-year-old son and two-year-old daughter were found next to him in the truck, all covered in blood. The father-of-five worked as a landscaper and had also recently left his previous job as a deliveryman for a grocery store.
The murder-for-hire was the first in the US on the mainland since 2004 and in the first attack on a Mexican.
Police said: “Cops have been contacted and the investigation has shifted to Houston, where they are gathering a larger team.
“The suspect is known to police as somebody who does a lot of work for various local gangs from Houston.”
Criminals are targeting Mexicans, especially in areas where crime is high, according to Mexican official Cesar Castillo, reported ABC.
The gunman may have feared seeing police approaching and he went for his gun because of his nervousness, the official added.
He added that police found a letter written in Spanish on the suspect’s body and said it was written by the gunman explaining that he had fired at three people he thought were robbers.
The letter also said Mr Dominguez had committed suicide. It was signed by the gunman’s father.
The FBI is now assisting in the investigation.
Image copyright Google Image caption The shooting was captured on CCTV
The state’s attorney’s office said he had been charged with murder in a first degree, but they had not received a full confession, reported ABC.
Dominguez, 39, is a native of Puebla, Mexico. He is thought to have moved to the US to start a new life in 2014 after several years living with his father.