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GREENSBORO, N.C. — A former Marine from North Carolina is in custody in Greensboro because a federal judge said he is a risk to society.

U.S. District Judge Patrick Auld decided to keep Kristen Douglas Welter, 54, in federal custody Wednesday for being a convicted felon in possession of a firearm. But Welter’s story goes far deeper than that one charge.

In 1986, Welter was convicted of trying to kill a state trooper in Beaufort, N.C., using pipe bombs. According to Asst. U.S. Attorney Graham Green, Welter rigged six pipe bombs surrounding the trooper’s car to explode when the car moved. The pipe bombs exploded, but the trooper survived.

Welter was sentenced to 25 years in prison. He served nine years and was paroled in 1995. He stayed out of trouble until just last month, when the FBI found out he had a .22 revolver in his home in Concord. It is illegal for a convicted felon to possess a firearm.

The FBI was already interested in Kristen Welter because of his expertise in explosives and his interest in remote control airplanes.

Welter served as an avionics specialist in the US Marine Corps. FBI agent Michael Davis said others had described him as a “mechanical genius, a MacGyver type” from the witness stand. Welter joined an RC airplane group in Concord called the Flying Aces.

The government presented evidence, claiming the planes Welter built and flew were extremely advanced. According to Davis, the planes had wingspans of 7-9 feet, flew as high as 4,000 feet, traveled at speeds of up to 100 mph and could carry a payload of up to 35 pounds.

Also, the government said Welter tested the planes by flying them over Charlotte Motor Speedway and J.B. Robinson High School. Davis testified that Welter had programmed one plane to swoop low over the school and announce “Primary objective reached” once it did so.

Davis told the court the FBI first began its investigation of Welter after he visited Lebanon in 2011 and told U.S. Customs and Border Enforcement agents that he may have had contact with members of Hezbollah. The U.S. government considers Hezbollah a terrorist organization.

Welter converted to Islam in 2006 and was active on a chat website called PalTalk.

Welter’s attorney, Ames Chamberlain, argued that Welter has not been in trouble since his 1986 conviction and has neither the means nor the ability to flee the country ahead of his court date. He argued for a form of supervised release, but Judge Auld said public safety would be at risk if Welter went free.

According to Chamberlain, Welter was charged with assault on a female in 2001, but the charge was dismissed.

Both sides said Welter received the revolver from a neighbor, and that he was worried about protecting himself in his vulnerable state. Welter had his knee replaced weeks ago and needed a cane to enter and exit the courtroom.

Judge Auld said the combination of Kristen Douglas Welter’s 1986 conviction, several inconsistent statements to FBI agents regarding his conversion to Islam and his visit to Lebanon made him a danger to the public and a flight risk.

Welter is currently being held in the Alamance County Jail.