Family members of Jazmine Barnes called on the local community to help catch her killer on Saturday, six days after the seven-year-old was shot dead in an apparently random murder her family believe is a hate crime.

The Harris county sheriff’s office has said it is working on a number of leads. It has released grainy images of a red pickup truck captured by a CCTV camera as well as a sketch of the suspect, thought to be a thin white male in his 30s or 40s.

“We’re going to find him no matter what corner we have to turn, no matter what rock we have to look under,” Jazmine’s mother, LaPorsha Washington, told a crowd of about 500 people at a rally, her voice cracking with emotion. “We’re going to find you, man.

“She was only seven. She was only seven.”

Other family members expressed thanks for the outpouring of support and appealed to the suspect to turn himself in. Part memorial, part call to action, the event was held in the parking lot of a Walmart close to the scene of the shooting. Some people carried balloons, flowers or teddy bears. Amid a heavy police presence, armed activists mingled with bikers and gun-control advocates.

“When you sit in your living room and you see this stuff on TV day after day after day, you take on it like it’s your own family member. And for a little child? Come on,” said Alma Thompson, 67, who has five daughters and was carrying a placard that said “stop the killing”.

Sheila Jackson Lee, a Democratic congresswoman from Houston, said federal resources including the FBI were being deployed to help local law enforcement.

“The heat is on and the heat is up,” she said. A spokesman for the Harris county sheriff said there were no fresh updates on the case as of Saturday afternoon.

Washington was driving with her four daughters to get coffee about 15 miles east of downtown Houston at roughly 6.30am last Sunday when a gunman in a pickup truck pulled up and opened fire. Washington, 30, was shot in the arm. Jazmine, who loved music and wanted to be a teacher when she grew up, was shot in the head. The family is black.

“We didn’t do anything wrong to this man,” Washington told reporters on Thursday. “This is something that I believe was a hate crime.”

The case has attracted national attention. The civil rights lawyer S Lee Merritt, who is representing the Barnes family, and the activist and writer Shaun King have raised a $100,000 reward for information leading to the suspect’s capture. As of Saturday afternoon a GoFundMe page had raised more than $67,000 for the family.

“A white male who was a complete stranger to them decides to target their family,” Merritt said at a news conference on Thursday. “We do believe that it was racially motivated in part because our nation at this moment is a highly racially charged society. We’ve seen a rise in hate crimes [according to FBI data] … this particular area of Houston is known for a racial element.”

Deric Muhammad, a local activist, suggested a possible link with another shooting that took place a couple of miles away in August 2017, when two black passengers were injured by a white man in a pickup truck. They survived but the attacker has not been caught.

DeAndre Hopkins, of the Houston Texans NFL team, pledged his $29,000 pay cheque from a playoff game to help with funeral costs.

“When I see Jazmine Barnes’ face, I see my own daughter,” the wide receiver said in a tweet. “On Saturday I will be playing in your honor, Jazmine”.