Training to work in maximum-security prisons includes stern warnings that add up to one message: Don't get too close to the inmates.

Never treat the relationship as anything other than professional. Never reveal personal details an inmate could use to compromise you. And never forget you are dealing with hardened, often cunning, criminals.

Investigators say prison tailor shop instructor Joyce Mitchell ignored those admonitions with frightening consequences, actually helping a pair of convicted killers make their power-tool breakout from upstate New York's Clinton Correctional Facility. She pleaded not guilty Friday night to charges of felony promoting prison contraband and misdemeanor criminal facilitation linked to the escape.

It doesn't have a catchy name like "Stockholm syndrome" — in which hostages become sympathetic to their captors — but the phenomenon of improper ties between inmates and prison staffers has been a touchy subject in corrections for years.

"It's embarrassing to the industry, it's embarrassing to management, but it occurs," said Michael Alexander, who spent over a decade working in different prisons and who has written about inmate and staff relationships.

Immersed in a world of inmates who can be excellent manipulators, some prison workers wind up doing inappropriate things for them out of compassion, greed, or romantic attraction, experts say. Other staffers are swayed by inmates' threats to harm their loved ones or expose the workers for breaking a rule.

"You spend more waking hours with them than your family, you get to know them, you see them age over the years," said Catarina Spinaris, executive director of Desert Waters Correctional Outreach, a training organization. "You have to be very cognizant of the need to maintain distance."

Inmates have all the time in the world to study prison workers for any vulnerability they can exploit, experts say.

Their "No. 1 objective is to make you forget they're an inmate," said Anthony Gangi, who has been working in corrections for over a decade and has a company that offers training in avoiding inmate manipulation. "You forget they're an inmate, you forget the role you have to play."

Terry Pelz, professor of criminal justice at the University of Houston and a former Texas prison warden, said newer employees tended to be more at risk than long-time employees, who "know the games inmates play."

At Baltimore's city jail, an inmate gang so corrupted and co-opted guards that its leader impregnated four of them, prosecutors said in a 2013 racketeering case against 44 people, including 27 corrections officers. Authorities said graffiti on a jail wall named 14 officers willing to have sex with inmates for $150 a tryst.

In another notorious case, convicted New York City cop killer Ronell Wilson impregnated a guard at a federal lockup. Their son — named Justus — was born a month after his mother's 2013 arrest for having the relationship.

The guard later pleaded guilty, with her lawyer saying she had "a misguided emotional belief that being impregnated by Ronell Wilson was providing him with a lasting purpose to his otherwise tragic and dysfunctional life."

In California, prison guards sent nude photos and love letters to inmates and an office technician had what authorities believed was a prisoner's child, the prison system's inspector general said in a 2012 report.

In Mitchell's case, she was investigated months before last weekend's breakout over a possible relationship with one of the escapees. The two were apparently separated for a while, but the investigation didn't turn up anything that warranted firing or disciplining her, District Attorney Andrew Wylie said.

Court documents say she smuggled hacksaw blades, chisels, a punch and a screwdriver bit to help the men escape.

Investigators also believe the 51-year-old Mitchell pledged to pick up the prisoners after their breakout but didn't show up, according to a person close to the investigation, who wasn't authorized to discuss it and spoke on condition of anonymity.

Mitchell's family has said she wouldn't have helped the inmates escape.

Experts say non-guard prison employees — such as vocational instructors, educators and health care providers — face particular challenges in keeping a "security mindset."

"When you're in a support service, you're somehow balancing the fact that ... you are to care about your patients and the patient population" with the realities of prison, said Lorry Schoenly, a nurse who consults on correctional health care issues.

And an employee can be made vulnerable by personal issues, including bad relationships and financial hardships, Spinaris said.

Spinaris suggests routine reminders about the dangers of letting professional boundaries slide, and creating an atmosphere that lets prison workers be honest when problems arise.

She said she had known some people who "ask for help and they manage to get through it, and some people were blacklisted and could never live it down."

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Associated Press writer Michael Virtanen in Albany contributed to this report.