After stopping for water near the US-Mexico border, one migrant was shot dead and another was wounded when they were fired on by the warden of an allegedly abusive Texas jail and his brother last week.

Michael Sheppard – the warden of West Texas Detention Facility, a privately-owned jail which once housed migrants detained by the federal government – and Mark Sheppard each face a charge of manslaughter after the 27 September shooting in rural Hudspeth county, roughy 90 miles (145km) from El Paso.

Prosecutors charged the Sheppard brothers, both 60, two days after the shooting. Michael Sheppard has since been fired from his job.

According to Texas’s public safety department, the victims in the case were among several migrants standing alongside a road drinking water from a reservoir when the Sheppards drove up in a truck. The migrants hid when the pickup first passed, but then the driver backed the truck up, got out, leaned over the hood and fired two gunshots at the group.

One of the group’s members, a man, was struck in the head and killed. Another – a woman – was struck in the stomach and injured before eventually being brought to the hospital, officials said. Neither of the victims’ names was immediately released to the public.

Investigators wrote in court records that witnesses reported hearing one of the men in the pickup hurl derogatory words at them and make the engine roar, the Associated Press reported.

Using a description of the pickup as well as surveillance cameras, authorities later found the truck and the Sheppard brothers.

The Sheppards – before they were arrested – claimed to investigators they were hunting at the time of the shooting.

A spokesperson for the West Texas Detention Facility’s proprietor, Louisiana-based LaSalle Corrections, later told media outlets that the company had dismissed Michael Sheppard “due to an off-duty incident unrelated to his employment”. The spokesperson wouldn’t elaborate, citing an “ongoing criminal investigation”.

The University of Texas and Texas A&M immigration law clinics and the immigration advocacy group Raices wrote a 2018 report that detailed multiple allegations of abuse – physical and verbal – against African migrants held at the West Texas Detention Facility.

The report alleges that the warden “was involved in three of the detainees’ reports of verbal threats [and] in incidents of physical assault”.

Authorities had trouble following up on the report’s allegations because many of those interviewed were soon deported, co-author Fatma Marouf told the Associated Press.

Though the report stops short of naming that official, Democratic congressmember Lloyd Doggett of Texas over the weekend confirmed that Michael Sheppard was the warden to which the 2018 report referred.

Doggett on Saturday joined other Texas Democrats in Congress in calling for a federal investigation in the shooting with which the Sheppard brothers have been charged.

“The dehumanizing, the demeaning of people who seek refuge in this country, many of whom are people of color, is what contributed to the violence we see here,” Doggett said.

Overall, in August, US authorities said they stopped migrants 203,598 times, an increase of 1.8% from 199,976 times in July but a decrease of 4.7% from the same month in 2021.

• The Associated Press contributed reporting