Keffer contends in another appeal that Diaz is entitled to a life sentence because that's what co-defendant Angel Toro received for second-degree murder in a plea deal.
The Supreme Court previously rejected that argument, but Keffer argues that was before new information recently was obtained - a sworn statement from jailhouse snitch Ralph Gajus.
In the statement Gajus recanted his trial testimony that Diaz, who spoke poor English, admitted he was triggerman through hand signs.
Gajus said he lied on the stand because he was angry with Diaz about leaving him out of an escape plan and police had promised to help him with his case in return for his testimony. Gajus later was sentenced to 20 years for second-degree murder.
His was the only testimony implying Diaz was the shooter. Other testimony suggested it was Toro.