A friend of the gunman who killed nine people in Dayton, Ohio last week allegedly told the FBI he helped the killer stockpile his arsenal—and even watched him assemble the murder weapon.

The FBI questioned Ethan Kollie hours after authorities say Connor Betts opened fire on Aug. 4. The high-capacity magazine, body armor, and part of the gun Betts used were all purchased by Kollie, according to a criminal complaint unsealed Monday. It is unclear if he had any prior knowledge of Betts’ plans.

Kollie, 24, is charged with an unrelated count of lying to purchase a gun for himself, authorities said Monday. He was arrested Friday and is being held in a Dayton jail.

During an FBI interview, Kollie said he bought and stashed the items for Betts at his apartment to help Betts hide them from his parents. Betts even came over to Kollie’s apartment to assemble a pistol several months before the massacre, according to the complaint.

Betts, 24, legally purchased the AM-15 pistol used in the attack, authorities previously said. The pistol has a shorter barrel than the AR-15 rifle variant, but uses the same ammunition and magazines. Attached to Betts’ pistol was the magazine Kollie purchased, capable of holding 100 rounds of ammunition. Betts also wore the body armor his friend bought.

Kollie told authorities he had known Betts since at least 2014, when they had used marijuana and acid together “four to five times a week.”

Despite his history with drugs—including a 2013 guilty plea for marijuana possession—Kollie allegedly answered “no” to a question about unlawful drug use on a form he filled out to purchase a handgun in May.

Kollie allegedly told authorities he lied because if “he told the truth about his drug use, he would not be allowed to purchase a firearm,” the complaint said.

FBI and ATF agents say they observed drugs in his apartment when Kollie consented to a search.

“While inside, the agents smelled marijuana and observed, in plain sight, paraphernalia consistent with smoking marijuana, including what appeared to be a ‘bong,’” an FBI agent wrote in the criminal complaint.

Kollie allegedly said he has smoked marijuana everyday for the last decade and “grew mushrooms” at his house that he “micro-doses” for energy.

Betts arrived in Dayton’s downtown Saturday night in his father’s car with with his younger sister, Megan, and another male friend, police said. About an hour after entering a bar with the duo, surveillance video captured Betts walking out by himself. About 45 minutes later, police said Betts returned to the bar and fatally shot his sister and wounded the friend outside.

Then, Betts opened fire across the street on a crowded line of people waiting to enter a bar. Police officers already deployed next to the bar almost immediately opened fire, striking Betts just before he entered the bar’s open door.

Authorities have not offered a motive for the massacre, but did say Betts had “violent ideations that include mass shootings and had expressed a desire to commit a mass shooting.” Former classmates and ex-girlfriends have said that Betts had violent attitudes going back a decade and was briefly suspended from high school after keeping a list of people he wanted to rape or kill.