Police have finally arrested a suspect in Dr. Spencer Tepe and Monique Tepe's murders ... with officials taking a man named Michael McKee -- Monique's ex-husband -- into custody.

The Winnebago County Jail in Illinois tells TMZ ... McKee was booked early today and is on an out-of-state hold.

We've obtained McKee's mug shot ... and, he's staring straight into the camera with no expression -- hair a mess and a pair of thick glasses on his face.

According to 10TV -- a local Ohio outlet which is citing court records -- police filed an arrest warrant for McKee after tracing his car on the night of the murders using surveillance footage.

Cops reportedly say the vehicle arrived in the area shortly before and left shortly after the killings. Online records say he's now facing 2 murder charges.

As you know ... the Tepes were killed in the early morning hours of December 30 and found by concerned friends hours later -- and, we shared the harrowing 911 audio with you which captured the intensity of the scene.

Police released video of a person of interest walking near the Tepes house around the time the two were killed ... and asked the public for any help identifying that person.

The Tepes had two young children -- a four-year-old and a one-year-old -- both of whom were unharmed and are staying with family members in the aftermath of the slayings.

Story developing ...