(CNN) — A US military veteran who admitted he faked being unable to walk for more than 20 years while claiming several hundred thousand dollars in disability benefits has been sentenced to prison time.

Christopher Stultz, 49, of Antrim, New Hampshire, was sentenced Monday to 18 months in prison after receiving more than $660,000 in disability benefits to which he was not entitled, the office of the US attorney for New Hampshire said in a news release.

Stultz had pleaded guilty to making false statements by faking the impairment that prevented him from walking to obtain veteran’s disability benefits, according to the US attorney’s office. Court documents show that for more than two decades, Stultz falsely claimed that he was unable to walk and needed a wheelchair to move around.

In addition to his prison sentence, Stultz was ordered to pay $662,871.77 in restitution, which is the total amount he is said to have gained from veteran’s disability benefits.

A year after joining the US Navy in 1995, Stultz suffered a spinal cord injury after falling off a horse. He was honorably discharged in 1999, according to court documents, and was rated as partially disabled by the US Department of Veterans Affairs (VA).

In 2003, however, Stultz claimed to the VA that he could no longer use both his feet. With that, the VA rated Stultz as 100% disabled and increased his monthly benefits. He was also given a total of nearly $190,000 in funding for special adapted cars to help mobility-impaired veterans, according to the US attorney’s office. Prosecutors say that Stultz never used the adapted cars and instead sold them for cash.

“This is one of the most egregious benefits fraud cases ever prosecuted in this state,” Jane Young, US attorney for New Hampshire, said in the release. “The defendant lied to the VA for 20 years to obtain disability benefits he was not entitled to. He stole not only from taxpayers, but also his fellow veterans who were entitled to the benefits.”

Prosecutors began investigating the case after receiving a report from “a concerned citizen,” according to the US attorney’s office. Court documents chronicle how Stultz was surveilled on multiple occasions and recorded walking without impairment.

In one example given in a sentencing memorandum, Stultz is seen using a wheelchair while inside a VA facility in Boston in October 2021. After leaving the VA, he stood up, lifted the wheelchair into his car and drove to a shopping mall. Prosecutors say that Stultz was then seen walking without impairment through multiple stores.

Investigators also interviewed people in Stultz’s life, including previous employers and acquaintances, who reported that “at work and in public the defendant was in good physical condition and walked normally,” court documents state, and that they had never known Stultz to use a wheelchair or other ambulatory devices as far back as the early 2000s.

During the investigation, Stultz underwent a psychological evaluation in which he was diagnosed with factitious disorder in 2023. The disorder is classified by the Diagnostic and Statistical Manual of Mental Disorders as a condition in which a patient falsely acts as if the patient has a physical or psychological illness.

According to court documents, Stultz has been in regular counseling since March 2020 along with physical therapy for several physical ailments. According to a sentencing memorandum filed by his attorneys, which asked the court for three years supervision instead of prison time, a psychologist opined that he “meets the criteria for factitious disorder and that it likely played a role in the alleged fraudulent behavior.”

CNN sought comment from Stultz’s attorney Dorothy Graham.

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