2:28 PM PT -- Law enforcement sources tell TMZ ... Diddy was never a suspect in the Tupac murder investigation, and that remains true today. In other words ... it's clear cops weren't buying what Keefe D was selling about Puff, and didn't deem his claim as credible.

The guy charged with Tupac's murder should not be released on bail, because he once alleged Diddy offered him 7 figures to kill 'Pac -- at least that's the case prosecutors are making to the judge.

Here's the deal ... Duane "Keefe D" Davis has been behind bars in Nevada since his arrest for the Tupac slaying, but he's asking the judge to let him out pending trial.

Prosecutors filed their opposition, arguing Davis should remain behind bars in part because of the 2009 statement he made to cops ... alleging Diddy had offered him $1 million to take out Tupac.

During the convo -- audio of which has been all over the Internet -- Keefe D told Vegas detectives Diddy had made personal contact with him and his cohorts saying he'd be willing to pay to have Tupac taken out by someone on the streets ... which Keefe says he took to mean him and his crew too.

Basically, Keefe D allegedly told cops he and his crew were hanging around Diddy one night when he said in front of a large group he'd give anything to see Suge Knight dead ... implying he'd pay top dollar for the murder without naming an exact amount.

Keefe also told cops ... after Tupac was murdered, he reached out to Diddy through a mutual friend to try to get paid, but he said he never did get the dough.

Prosecutors argue, at the very least, the admission by Keefe shows he's the kind of person who would consider murdering someone for cash -- and, therefore remains a menace to society in their eyes.

Diddy has, on multiple occasions, denied he had anything to do with Tupac's murder or that he offered money for someone to take him out. Also worth noting ... there's no indication that there's any kind of active investigation into Diddy based on Keefe's years-old claims.

The judge has yet to rule on Davis' release.