BOULDER, Colo. -- A Longmont man has pleaded guilty to three felony counts of possessing explosives in a plea deal that will spare him jail.

Ronald Swerlein, 51, pleaded guilty Thursday in Boulder District Court.

The retired electrical engineer was originally charged last year with 10 counts of explosive possession after police responded to complaints from neighbors and removed 400 chemicals, including volatile explosives, from his home. Swerlein told police he was trying to develop a model rocket fuel.

Neighbors called police after they reported hearing three explosions at the house.

According to an arrest affidavit obtained by 7NEWS, Swerlein called those explosions in his garage "tiny, tiny tests" he was doing to teach himself about rocket fuel technology. The affidavit says Swerlein told investigators that he "believed the experiments in his house were the safest thing because they were enclosed in his garage. He always made the samples small and covered them with tires."

Authorities said they found homemade nitroglycerin and MEKP, a volatile chemical that had been stabilized and hidden in the basement.

Officers said they also found model rockets, rocket kits and engines, a launching pad for model rockets, books on homemade explosives, expensive chemistry glassware, a handwritten "to-do" list and warning note, flare guns, a Taser weapon and three metal grenade shells.

They also reported finding magazines from the Animal Liberation Front, which the FBI considers to be a terrorist group.

Under his plea agreement, Swerlein will be sentenced to six years of probation at a hearing set for Aug. 8. He would have to perform community service, undergo a mental health evaluation, and he would be barred from possessing any explosives.