The Man in the Green Chevy — Susan Rogers Cooper

Meet Milt Kovak, divorced, overweight, mid-40s Chief Deputy of Prophesy County, Oklahoma.
When his sleepy little burg and the surrounding towns suddenly get some horrific murders, Milt's cool intuition and great people-skills are put to the test.
Someone is raping and killing old ladies of the county, truly heinous crimes.
His only witness,a married Laura Johnson with 3 children and a frequently absent husband, professes to seeing a green Chevy in the vicinity of the murder scene.
As Milt begins his search, he quickly finds himself drawn to beautiful Laura and her wonderful kids and finds himself falling in love with her (as if matters weren't bad enough!)