Oct. 25, 2011 -- A Texas serial rapist who is attacking older members of one of the nation's most prominent sororities appears to have bizarre and dangerous compulsions, a forensic psychologist told ABCNews.com.
The rapist has assaulted four women in the Dallas area, all in their mid-50s to mid-60s who are alums of the largest Greek-lettered black sorority in the world, Delta Sigma Theta.
"Most often the target population is going to be a younger group of women," said Naftali Berrill, director of the New York Center for Neuropsychology and Forensic Behavioral Science.
He described the case as "weird and unusual."
"It's typically not going to be middle-aged women who come from a certain subclass," Berrill said.
"Someone with this obsession doesn't cultivate these obsessional ideas overnight," he said. "This is coming from, I'm guessing, something much more historical and deeply rooted in this guy's psychological history. It's just too specific a group."
The age of the women is so atypical that Marisa Mauro, a forensic psychologist who owns a private practice in Austin, Texas, told ABCNews.com, "I have not heard of serial rapist who has targeted that population."
Another oddity: the crimes took place in three different cities over an 11-month time period.
Sorority Rapist's Bizarre Compulsion
"Rapists don't usually target specific people for any particular reason," criminal profiler Pat Brown said today on "Good Morning America." "They usually find victims who are just in their neighborhood who are convenient."
The Dallas-area rapist has attacked between 9 p.m. and 4 a.m. at the homes of the victims, who were each alone at the time. Two of the attacks happened in Plano, Texas, one in Coppell and the most recent assault was on Oct. 14 in Corinth.
"The one thing I can say with certainty is that there will be an obsessional component to this guy," Berrill said, but "his motivation for doing this remains to be seen."
Police have identified a suspect on surveillance video, a black man who appears to be in his late 30s to mid 40s, about 5-feet-7 to 6-feet tall, and between 250 and 300 pounds, according to Plano police spokesman Andrae Smith. He may also have a thin or trimmed beard with a receding hairline.
"We believe the people responsible for assisting us and catching this individual will be the public," Smith said. "We have to learn the nature of his association with the group itself and the victims."
"According to our investigators he has indicated that he knew information about the victims," Smith said, but detectives are puzzled as to how he obtained this information, or why he targeted these four women.
"We don't have any indication to believe our victims know the suspect," Smith said. "We don't know the nature of what he knows or the information that he has."
Berill said the rapist is "obviously doing his research and doing some stalking," and it's likely he has been "obsessing about them and has determined that this is the way he can humiliate and pay them back for perceived humiliation, insults, or rejections."
The victims are closer to his mother's age than a girl he would have dated, Berill said, which makes for "an interesting dynamic."
"He's targeting women that might be his mother's age," Berill said, suggesting he might have had issues with his mother, "some dynamic going on with his father or a male in his life."
"He's obviously not looking to disrupt college girls. He's disrupting what is a middle-age, middle-class lifestyle and wreaking this havoc on these women who are not out partying, not being available to him in a social sense. That's what makes it so curious," Berill said.
It's also possible that class is an issue, and the rapist's mindset could be: "Let me inflict harm on this elite population. Let me show them they're not so hot or so special," Berill said.
Sorority Members Advised to Hide Affiliation on Facebook, Cars
The Plano police department met with Delta Sigma Theta's local chapter Thursday. Now the sorority's national office in Washington, D.C., is advising its Dallas-area members to avoid identifying their affiliation with the group.
In a statement released Thursday, the organization said local members should "remove Sorority paraphernalia from their vehicles, key chains, homes and offices" and "refrain from wearing clothing or accessories that identify them as members."
The sorority also advised alums to be avoid sharing information about their whereabouts on social media websites such as Facebook and Twitter, warning members of the Dallas-area chapters to "be aware of their surroundings, ensure that their homes are locked and secure at all times, and avoid going out or staying home alone if possible."
And that's exactly that kind of fear the rapist seeks to cultivate, Berrill said.
"The community itself is on red alert, so everyone is upset," he said. "There's a lot of power in being able to upset a whole locale. Now everybody is looking and wondering."
Anyone with information about the suspect shown in the surveillance video is asked to call the Plano Police Department Tip Line at 972-941-2148 or the Crime Stoppers Tip Line at 877-373-8477.