Lt. Col. Tony Shaffer believes North Korea’s repeated use of drugs killed American student Otto Warmbier.

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“He was tortured not so much the way—pulling fingernails out with tools but by repeated drugging—sodium pentothal,” Shaffer told the FOX Business Network’s Maria Bartiromo.

The retired U.S. Army general said North Korea was trying to figure out whether Warmbier was a CIA spy and repeated exposure to drugs during that year in captivity is what caused brain damage.

“I think that during his confinement they did it over and over and I think that was one of the reasons that they released him,” he said. “But no doubt they kind of like saw that some things are going to go tragically wrong, we better get this guy out of here anyway so it served their purpose to get him out.”

In Shaffer’s opinion, there needs to be “some level of retaliation,” from President Trump.