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.

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.