DALLAS (AP) — A Dallas-area rapist appears to be preying on members of a national black sorority, leading the organization to urge alumnae to remove any trace of their affiliation from cars, clothing and even their key chains.

Delta Sigma Theta issued the warning this week, citing four sexual assaults, all involving black women in their mid-50s to mid-60s. Police say the assailant indicated during the attacks that he knew personal information about the victims.

“We believe it’s more than just accidental,” said Matthew Kosec, deputy police chief in Coppell.

Cynthia M. A. Butler-McIntyre, national president of the sorority, said it isn’t certain that the victims were targeted because of their sorority affiliation, but “we are erring on the side of caution” and advising members “to take the necessary precautionary measures.”

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The group urged members to avoid displaying any items identifying them as sorority alumnae, including vehicle stickers, jewelry, clothing and accessories. They also warned members to remove information such as their whereabouts from social networking sites.

Detectives have not determined exactly how the rapist might be learning about the sorority affiliation.

“We just don’t know if the suspect is identifying these ladies as they are out shopping in the area or if it’s something more advanced than that” such as using social networking sites, Corinth police Capt. Greg Wilkerson said.

The most recent attack was Oct. 14 in Shady Shores, said Corinth police, who are investigating the rape in the nearby community. The Coppell attack occurred Sept. 15.

The other two assaults took place in Plano — one in April and another “prior to that,” said Plano police spokesman Andrae Smith, who would not elaborate on the earlier date.

The attacker is described as a black man in his late 30s to mid-40s, 5 feet 7 inches to 6 feet tall and weighing 250 to 300 pounds. Police in Plano released a video shot in April from a surveillance camera showing an unidentified man who appeared to resemble the description. Authorities say they would like to question that man in relation to the attacks but declined to provide more details.

Smith, who said the victims did not attend the same college, said investigators noticed the similarities after the second attack and shared the information with surrounding cities.

“The pattern of the alumnae membership was the big flag that put this together,” Kosec said. “When you have a sexual assault like this, the detectives are very good about getting all sorts of details that could lead to the suspect.”

Delta Sigma Theta counts more than 200,000 mostly black college-educated women among its members. Seventy-six percent of the group’s members are alumnae, while 24 percent are still in college. The group has more than 900 chapters located around the world.

Copyright 2011 The Associated Press.