More than a month after muralist Matt Callahan died of head injuries sustained near a Tampa bar, police are still working to determine what happened.

But in a flier posted this week on Facebook, his father publicly accuses a man of killing Callahan and offers a $5,000 reward to anyone who can prove it. By Wednesday, the reward had climbed to $8,000, boosted by a restaurant owner who twice commissioned the artist's work.

"I think the homicide detectives are doing a great job and I have no complaints," said Callahan's father, St. Petersburg lawyer Michael Callahan. "But I think they could use a little help. There are some people who know things but are reluctant to come forward."

The Tampa Police Department has not named a suspect or a person of interest in the case, or even said that the artist was murdered, only that he argued with a man the night of his fatal injury.

The agency wants people to provide information on all crimes, but pointing a finger at this stage without police confirmation can be "problematic" to an investigation, according to TPD spokeswoman Janelle McGregor.

RELATED: Epilogue: Muralist Matt Callahan 'lived 100 percent, 100 percent all the time'

Police found Callahan on his back with head trauma just after 3 a.m. on April 11 in a median on Gandy Boulevard in Tampa outside the Warehouse Liquor Store and Bar.

He died on April 13 at Tampa General Hospital. He was 40 and left a wife and a 5-year-old son. His murals adorned walls throughout the Tampa Bay area, including at the Lure restaurants in Tampa and St. Petersburg, inspiring owner Tom Golden to contribute $3,000 to the reward fund.

Callahan's friends and family took to social media and said he had argued with 34-year-old Keith Mauga of Tampa in the Warehouse the night he was injured.

Mauga's attorney, Dirk Weed, later confirmed to the Tampa Bay Times that he had been in contact with "several members of the police department" because witnesses indicated his client was involved in an altercation with Callahan.

The Times could not immediately reach Weed on Wednesday to respond to the father's flyer, which has been shared more than 230 times on Facebook.

BACKSTORY: Attorney says police investigating muralist Matt Callahan's death are asking about client.

Michael Callahan wrote the flyer after talking to people who were at the bar that night, he said.

He wrote it as if his son were the one telling the story and the one accusing Mauga, alleging that he "approached my truck outside The Warehouse bar on Gandy Blvd. and struck me so hard that the back of my head and brain were severely damaged."

"Three minutes after Mr. Mauga struck me, I was found in the median of Gandy Blvd. where Mr. Mauga and perhaps one or two of his friends left me. Within a few minutes, the EMT's found me with fixed pupils and no chance of recovery."

The flyer, with old jail booking mugs of Mauga, goes on to detail his criminal history, including a 2017 arrest relating to the aggravated battery of a patron outside a St. Petersburg bar. That argument started over a game of pool, a court record states. A jury trial is set for June 6.

Michael Callahan said he learned that Mauga was angry with his son over a pool game too.

The flyer provides personal information on Mauga, including two Tampa addresses, a location where he trains as a boxer and another where he sometimes resides in New Jersey.

"I want him to understand I am watching his every move," Michael Callahan said. "I want him to understand clearly from me as a father that I am not afraid of his threats or conduct."

The flyer concludes by inviting people with information about Matt Callahan's death to call TPD homicide detective Nicholas Sclavakis at (813) 276-3556 or leave an anonymous tip at Crime Stoppers of Tampa Bay, (800) 873-8477.

Contact Paul Guzzo at pguzzo@tampabay.com. Follow @PGuzzoTimes.