(CNN) -- The FBI has joined New Orleans investigators in their hunt for a carjacker who shot a good Samaritan who died in front of his young sons, police said Thursday.
"Agents contacted New Orleans police yesterday to offer their help, and detectives accepted the offer because of all the FBI brings to the table," a New Orleans police statement said.
Police provided further details of Wednesday's shooting, saying Harry "Mike" Ainsworth, 44, was shot in the chest after he jumped on a vehicle's hood to help a woman being carjacked.
After being shot by the carjacker, Ainsworth "ran a few feet from the car, collapsed in someone's front yard and died on the scene," the police statement said.
The gunman left the car and ran, police said.
"We are grateful that a number of people in this neighborhood approached police to tell us what they saw," said police Supt. Ronal Serpas. "Because of their eyewitness accounts, we were able to put together a composite sketch of the man we believe committed this unthinkable crime.
"Mr. Ainsworth was a longtime supporter and volunteer to the NOPD's COPS 8 program, which provides assistance to the officers of the 8th District. The members of the 8th District station sincerely appreciated his efforts to fix up the station by painting and repairing various things in the building. He'll be missed," Serpas said.
The murder marked New Orleans' 20th homicide for the year -- up from 17 at the same time last year, police said.
Ainsworth was with his two sons, ages 9 and 11, while they were waiting for a morning school bus, Ainsworth's brother, Bill, said.
The two boys saw their father try to help the female motorist in the Algiers Point neighborhood and, after their father was shot, the boys ran to him and sat with him until emergency responders showed up, Bill Ainsworth said.
The two sons were with their father when he died, Bill Ainsworth said.
Meanwhile, the family was struggling with the loss of Harry Michael Ainsworth, and the publicity surrounding the event has prompted widespread donations to the family.
Ainsworth's girlfriend, Cheryl Conner, 33, who's also the mother of the two boys, woke up Thursday morning and leaned over Ainsworth's empty leather jacket and slippers, said Billy Ainsworth Cole, sister of Mike Ainsworth. The couple have been together for 12 years, Cole said.
Conner broke down, crying in a fetal position, said Cole, who's now staying with Conner.
"It's so hard, it's so hard right now being the backbone in the family right now. I break down every now and then, but so many people in this community are hurting," said Cole, 39.
"Just to know that my nephews were leaning over their daddy and watched him die... it's devastating."
The fact their father will never be around is starting to sink in for the two boys, Cole said.
"The 11-year-old was really distraught yesterday, but the 9-year-old seemed to be just angry and holding it together surprisingly after something like this occurred," Cole said Thursday. "This morning, the 9-year-old was holding back his emotions and was angry until someone pulled out a photo album, and he just came broke down crashing and just let it all out for about 45 minutes."
Cole has been unable to find one of her brothers to tell him of the family loss.
"It's rough right now because we still have a brother, Phillip Patrick Ainsworth, who is missing," Cole added. "He suffers from paranoid schizophrenia and disappears at times and right now he's the only member of the immediate family we can't find."
Mike Ainsworth's heroism has prompted donations to a fund assisting his sons and family, a spokesperson close to the family told CNN.
"You wouldn't believe the response. The response is overwhelming. People from Canada and all over the world have been calling the bank," the spokesperson told CNN. "If you knew him, you would be crying. He always did things for people."
"The Algiers Point community is a very secluded community and everyone knows each other. He would always help someone, free of charge. He wouldn't even ask for a handout. The whole community here is in a very somber mood," the spokesperson said. "He didn't even think about what he was doing, he just went to her aid."
"Everyone is just in mourning for this tragedy," said the spokesperson.
The series of events began when a man approached a woman motorist sitting in her vehicle, knocked on the window and asked for the time, New Orleans police said.
As the woman turned to check the time, the man pulled out a handgun and told the woman to get out of the automobile, police said.
Ainsworth then jumped on the hood of the vehicle, and the carjacker shot him, police said.
Police released a sketch of the suspect and were offering a $5,000 reward to anonymous tipsters who can help detectives find a black man, in his 20s, between 5-6 and 5-8 in height, and weighing 140 to 150 pounds, detectives said.
The woman was unharmed, New Orleans police spokesman Frank Robertson said.
A fund at the Algiers Branch of Whitney Bank has been established to help the Ainsworth family.