Wisconsin School Shooter's Alleged Co-Conspirator Was Creepy, Ex-Neighbor Says

An ex-neighbor of the California man accused of plotting a local mass shooting in a coordinated attack with the Wisconsin school shooter is speaking out ... telling TMZ about the chilling, ominous energy he always carried.

The woman, choosing to remain anonymous, talked to us on "TMZ Live" Thursday, revealing that during the 5 to 6 years she lived near 20-year-old Alexander Paffendorf, he exhibited seriously creepy behavior -- never talking, avoiding eye contact, always wearing his hood up, and other unsettling anti-social habits.

Catch the video, because she dives into the specific moments when Paffendorf made her feel super uncomfortable around their apartment complex -- and it’s spine-chilling.

In fact, his behavior was so bizarre, she claims it was even brought to his parents’ attention during one particularly eyebrow-raising incident -- but they didn’t do anything about it.

But, it seems there was plenty happening behind closed doors -- she says another neighbor who lived directly below the Paffendorfs called the cops last year after hearing a disturbance, something she goes into more detail about in our full interview.

Bottom line -- she says neighbors would echo similar unease about Alexander ... and when he was detained, it didn't come as a shock to them.

As we reported, Paffendorf was detained after being accused of exchanging messages with Natalie Rupnow about opening fire at a local government building ... while she attacked the Abundant Life Christian School in Madison on Monday, killing 2 and wounding 6.