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PLANO - Police are looking for a man wanted in a series of sexual assaults with one common factor: how the victims know each other.

The four attacks happened in Plano, Corinth and Coppell.

In surveillance video released by Plano police, a man is partially seen that investigators think is entirely responsible for four sexual assaults over 11 months in the three cities.

Plano Police spokesman David Tilley said the last assault happened a week ago Friday.

What's worse is that it's happening in the one place they feel the safest, and that is in their own home, Tilley said.

After investigators learned all the victims are African American women in their mid 50's to mid 60's, they found another startling common thread.

All are alumni of the Delta Sigma Theta sorority.

Our two victims were associated with a particular sorority alumni group, Tilley said. After we had the additional offenses in the other cities, we were able to determine that their victims were also associated in the same way.

The Dallas alumni chapter is notifying members.

Police said the suspect indicated he knew information about his victims.

And they urge anyone with information to call Plano, Corinth or Coppell police.

The suspect is described as a black male, in his late 30's to mid 40's. He's 5'7 to 6'0 tall and 250 to 300 pounds. He has a heavy build with a thin, well-trimmed beard and a short haircut or possibly balding or receding hairline.

E-mail bwatson@wfaa.com

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