Baseballer Chris Lane was gunned down and left to die while jogging in Oklahoma in 2013.
But after just two years behind bars, James Edwards has been released.
Mr Lane’s mother Donna said she was shocked by news Edwards was now free.
“We had always been assured it was 15 years,” she said.
“I just miss him (Chris).”
Edwards was sentenced to a maximum of 25 years in jail after pleading guilty to being an accessory to the senseless drive-by murder.
“It doesn’t change anything for me,” Ms Lane said.
“I don’t get to see my boy.”
Mr Lane was jogging when he was randomly shot in the back.
Edwards was just 15 at the time of the shooting.
He now has to wear a GPS tracking device, but District Attorney Jason Hicks believes he should still be behind bars.
The man received credit for time served and despite the expectation he would be behind bars until December next year, Oklahoma’s Department of Corrections allowed Edwards out on its GPS monitoring device program on Thursday.
It is believed he has entered a sober living facility and is being monitored 24-hours-a-day, seven-days-a-week.
“As I understand, he was a very well-behaved inmate,” Oklahoma Department of Corrections public information officer Matt Elliott told AAP on Friday.
But Mr Hicks, who prosecuted the case, was not told about Edwards’ release until after he got out.
“This case is one that will bother my staff and I for the remainder of our lives,” Mr Hicks said.
“It tore apart a family and a community.”
Mr Lane’s mum said she thought the District Attorney felt as though they had let her family down.
“I said, ‘Well, if you didn’t know, there’s not much you could do’,” she said.
Mr Lane’s legacy lives on in Melbourne.
The Chris Lane Memorial junior baseball field will be built in Strathmore, in Melbourne’s north-west, in 2019.
It is where he played baseball before moving to the US.
Chancey Luna, who fired the shot, and driver Michael Jones are both serving life sentences for Mr Lane’s murder.