FRED AVERY JR. is a convicted murderer with a long, violent rap sheet, so it's unlikely that many people were surprised when he allegedly stabbed three prison guards at a city jail last Monday as they tried to subdue him after he fought with his cellmate.

But what did surprise some was that Avery was still in jail - and not in a mental institution in Norristown.

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Arrested 19 months ago, the 49-year-old Ogontz man remains in a legal limbo because of his mental status. He's been declared incompetent to stand trial - but has been waiting for nine months for space to open up at Norristown State Hospital, where he must be treated before his competency can be re-evaluated.

Cases like Avery's are not unusual. Twelve percent of the 8,500 inmates in Philly's six jails are considered "seriously mentally ill," while another 4 percent are on the system's behavioral-health caseload with less-serious mental-health needs, said Bruce Herdman, chief of medical operations for city prisons.

Avery joins more than 50 inmates on a waiting list for beds at Norristown State Hospital, while a 66-bed licensed psychiatric hospital at the Detention Center and another 430 "transitional" beds in several prisons are almost always full, Herdman added.

Common Pleas President Judge Sheila Woods-Skipper, who oversees the system's mental-health court, said she has lobbied for years for more attention and funding to alleviate the problem.

"There are not enough treatment facilities, there is not enough housing, there are not enough counselors" for mentally ill people, Woods-Skipper said. "The prisons can't continue to house them all."

Mentally ill inmates pose unique management problems for prisons, experts agree.

Many find it tough to follow jail rules, and they are twice as likely as other inmates to incur infractions, according to the Treatment Advocacy Center, a Virginia-based nonprofit agency advocating for better mental-health treatment and policies.

Although incidents involving force in city jails have risen in recent years, Herdman and Prisons Commissioner Louis Giorla said they couldn't immediately provide numbers on how many of the 800 use-of-force incidents in Philly jails last year involved inmates with mental issues.

Mentally ill inmates tend to linger in prison longer - for competency issues, disciplinary infractions or other reasons, Herdman said. The average Philly inmate spends 90 days in jail, while seriously mentally ill inmates average 115 days behind bars, Herdman said.

Inmates with mental issues also bounce in and out of jail more often than other inmates, experts say. Philly's inmates without mental issues average 4.7 incarcerations, while those deemed seriously mentally ill average seven, Herdman added. The system deems inmates seriously mentally ill if they have schizophrenia, major depression or schizoaffective, brief psychotic or bipolar disorders.

Kidnapped kin?

Cops arrested Avery in April 2013 after he allegedly kidnapped his father and sister and drove them an unspecified distance while threatening to kill them with a loaded shotgun, according to court records.

Prisons officials said they couldn't disclose the nature of Avery's mental issues with the Daily News, and court records didn't specify his illness.

But court records show that Avery's alleged stabfest last Monday was far from his first violent incident:

August 1992: Police arrest him for shooting a man in the buttocks inside the man's home. He is convicted and sentenced to two to five years in prison.

May 1993: He shoots a man named Leroy Simpkins, 23, in the chest during a road-rage incident in Logan. Simpkins also is armed and approaching Avery's vehicle when shot, prompting a plea deal in which Avery is sentenced to 7 1/2 to 15 years in prison for third-degree murder.

2013: He is disciplined for stabbing another inmate at Curran-Fromhold Correctional Facility, according to Lorenzo North, president of District Council 33 Local 159, the union representing city correctional officers. The incident is among four "critical" infractions Avery incurred in the past year, Giorla said.

In last Monday's incident, Avery allegedly used a sharpened screw he'd pried from his cell toilet to stab a correctional sergeant and two officers at the Philadelphia Industrial Correctional Center.

The sergeant suffered cuts to his face and eye. One officer was stabbed in the back and head, and another officer in his hand.

In the ensuing scuffle, Avery suffered a fractured jaw for which he was treated in the intensive-care unit of Aria Health's Torresdale hospital.

Shawn Hawes, prisons spokeswoman, said that Avery has been released from the hospital and returned to prison. Charges are pending.

On Twitter: @DanaDiFilippo