A drug buddy of Dayton shooter Connor Betts bought body armor and gun parts for the killer — including the 100-round magazine the gunman used in his deadly rampage, prosecutors claimed Monday.

Ethan Kollie, 24, purchased the armor, the “double drum” magazine and the upper assembly of an AR-15-style assault rifle for Betts — then kept them in his own apartment “in order to assist Betts in hiding the items from Betts’ parents,” federal prosecutors allege.

Kollie — who admitted that he and Betts did “hard drugs” four to five times a day in 2014 and 2015 — even helped the mass murderer assemble the firearm he used in the massacre, the feds charge.

They put the rifle together in Kollie’s apartment about 10 weeks ago, officials said.

Kollie was questioned just hours after the mass attack, and again several days later.

He was arrested Friday on charges he lied on federal firearms forms, after admitting to investigators that he smoked pot regularly for the last decade even though he wrote on a federal firearms form that he did not use marijuana or other drugs.

Kollie is charged with making a false statement regarding firearms forms, as well as possession of a firearm by an unlawful user/addict of a controlled substance — and could face up to 15 years in prison if convicted.

None of the charges are tied directly to the mass shooting, nor is there any indication Kollie knowingly helped Betts plan or carry out the slaughter, officials said.

“Mr. Kollie does not stand accused in taking part in planning the shooting,” US Attorney Benjamin Glassman said Monday. “There’s no allegation of that. That’s not what this case is about. What this case is about is anyone who is discovered to have criminal culpability, it’s not something federal agents who are investigating will ignore.”

Kollie was arrested Friday in Beavercreek, Ohio, with the complaint unsealed Monday in Dayton federal court.

Betts, armed with an assault rifle, opened fire in Dayton’s Oregon District, a trendy nightspot neighborhood, early Aug. 4, killing nine people in the 30 seconds before police officers shot him dead.

The attack came one day after Patrick Crusius walked into an El Paso, Texas, Walmart and allegedly shot 22 people dead with an assault rifle.

Meanwhile, a new report revealed Monday that Betts had to drop out of college in 2014 because he didn’t “have the maturity” to stick it out, according to the Cincinnati Enquirer.

“I write to inform you that I must withdraw from Wright State [University], as I do not yet have the maturity to handle college,” he wrote to his adviser at the time.

Betts’ unofficial transcript indicates he was enrolled in the psychology department.

Kollie’s lawyer, Nick Gounaris, said his client was “shocked” by Betts’ actions but declined to comment about the charges.