Dong's attorney says he entered the plea in Milford Superior Court on state charges, including illegal possession of an assault weapon and breach of peace.
Federal charges of unlawful transport of a semi-automatic Bushmaster rifle were also lodged last week against the 23-year-old Fairfield man.
Dong was carrying two loaded handguns on the West Haven campus Dec. 3 and had the Bushmaster and bullets in his car parked nearby, police have said. The campus was locked down more than four hours.
Police say they found 2,700 rounds of ammunition and newspaper clippings about the Aurora, Colo., theater shooting in Dong's bedroom in his Fairfield home.
“I don’t know what it was, but something bad was going to happen either yesterday or sometime in the future,” Fairfield Police Chief Gary MacNamara said last month after the search of Dong's home on Stratfield Road.
Read the full story here at the NBC Connecticut website.
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