Police are continuing to test DNA in the investigation into attacks on members of a national black sorority.
A man has attacked at least four alumnae members of Delta Sigma Theta in home-invasion-style attacks since November 2010, sexually assaulting three of them. Women in Plano, Coppell and Shady Shores have been attacked.
Detectives said 10 to 12 DNA samples are currently at a lab for comparison to the attacker's DNA.
But police said it's becoming more difficult to get men to submit to DNA testing because of fears about how the sample would be used and if it could be entered into a database.
"I understand the hesitance that someone might have as far as giving the DNA sample," Plano police spokesman Officer David Tilley said.
But police said each sample is only used for comparison in this case and then destroyed.
"The sample that we receive, we do a comparison check against the one that we have of the suspect," Tilley said. "Once that's done, it's disposed of. We don't continue to keep it. We don't put it in the computer system for CODIS to keep it. It's disposed of, and it's done."
Investigators urge all men contacted by detectives to submit to the testing so they can be eliminated as potential suspects and police can narrow their focus.
Police described the investigation having peaks and valleys. Tips pour in when there is attention on the case but go dormant when there isn't, police said.
"At first they came in pretty hot, then they stated slowing down, so it's kind of been up and down," Tilley said.
Police have circulated photographs and video of the attacker in hopes of getting information that could identify a suspect.
"That's what we believe is going to wind up solving this -- somebody's going to call in," Tilley said.
Investigators have done DNA testing on about two dozen men. Other tips were eliminated based on photographs, body markings, alibis and other information.