An 81-year-old man who spent decades behind bars for a series of bank robberies said he stole from an Arizona credit union months after being released from prison because his monthly $800 Social Security payments were too small to live on, according to court records.

Robert Francis Krebs walked into the Pyramid Federal Credit Union in Tucson in January 2018 without a disguise and allegedly put a BB gun -- that looked like a handgun -- on the counter and demanded cash. He told FBI agents he "kind of wanted to get caught" so he could return to prison, court documents said.

Lawyers for Krebs, who turns 82 in July, say their client has reported having symptoms of Alzheimer's disease. A neuropsychologist concluded Krebs was not competent to stand trial because he has dementia. Two other experts have accused him of embellishing his condition to avoid prosecution.

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A judge will determine if Krebs is competent to stand trial.

“They really can’t make it on the outside,” retired FBI bank robbery expert William Rehder, who is not involved in the case against Krebs, said. “He is right — Social Security is probably not enough to keep him going, and he has no prospects for income.”

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Krebs served more than 30 years in prison for a 1981 bank robbery in Florida where he took $8,300, and was sentenced to three years for a 1966 embezzling conviction in Chicago. Authorities said he stole $72,000 from a bank where he was employed as a teller.

He served another 17 years for theft and robbery convictions in Arizona. He had been out of prison for six months when he allegedly committed the Tucson credit union robbery. He took $8,400 and was arrested at a hotel and has been in police custody since. He was charged with bank robbery for robbing the credit union and has pleaded not guilty.

Krebs' attorney, Leonardo Costales, said in court records that there is not enough evidence to show his client is competent to stand trial.

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J.L. Hunter “Red” Rountree, who has been dubbed America’s oldest bank robber, was convicted of committing robberies at banks during the late 1990s and early 2000s in Mississippi, Florida and Texas.

Rountree, who later died in prison at age 92, committed two robberies in his mid-80s and was 91 at the time of his last heist.

The Associated Press contributed to this report.