Two women in Mississippi have pleaded guilty to a hate-crime conspiracy to attack black people in Jackson that culminated with the June 2011 murder of James Anderson, who was beaten up and run over with a pickup truck.

The guilty pleas of Shelbie Richards and Sarah Graves bring the total number of white teenagers who have confessed to being involved in the conspiracy to nine. Together they violated federal hate crime laws by launching a spree of racially motivated attacks on vulnerable black people.

Deryl Dedmon has already been sentenced to two concurrent life sentences, as the driver of the Ford F250 truck that was used to kill Anderson. Six other men have yet to be sentenced for their roles in the conspiracy.

Anderson was murdered early on a Sunday morning, after two car-loads of white teenagers set off from their homes in a mainly white area of Rankin County to predominantly black Jackson. As they headed for Jackson, Dedmon reportedly shouted: “Let’s go fuck with some niggers.”

When they came across Anderson, 49, in a parking lot outside a hotel, they shouted racial slurs including “White power!”

Several of the teens attacked Anderson, and then Dedmon drove over him with the pickup truck.

Unknown to the group, the events were caught on a surveillance camera, footage that quickly led police to the perpetrators.

Richards and Graves, now aged 21, have admitted encouraging others in the group to set off in search of black people to assault. They face maximum sentences of three years in prison and $250,000 fines.

Richards also pleaded guilty to encouraging Dedmon to run over Anderson and then lying to police over the incident.

Vanita Gupta, President Barack Obama’s nominee to lead the civil rights division of the Department of Justice, told the Clarion Ledger that “the continuing investigation into the events surrounding the vicious murder of James Craig Anderson that resulted in today’s guilty pleas demonstrates that the Department of Justice will vigorously pursue justice for every victim of racially motivated violence”.

The death of Anderson was the tragic nadir of a series of race-hate attacks committed by the group. In several such assaults, they used beer bottles, slingshots and their cars in attempts to harm vulnerable black people. Their targets were selected for being homeless or drunk, in the expectation that they would be less likely to report what happened to them to police.