The mom of one of two missing Iowa cousins agreed to submit to a second polygraph test Monday to show she is again cooperating with investigators searching for the girls, their aunt said.

Misty Cook-Morrissey was going through the so-called lie-detector test Monday with state and federal detectives at an elementary school in Evansdale, her sister, Tammy Brousseau said. Misty is the mother of 10-year-old Lyric Cook-Morrissey, who vanished with her 8-year-old cousin Elizabeth Collins on July 13 while riding bikes near a lake in the northeastern Iowa town.

Cook-Morrissey had submitted to one polygraph last week, Brousseau said, but had announced she would not take any more after complaining she had been harshly interrogated and accused of involvement in the disappearance. That stance caused some friction among family members, Brousseau said, and in recent days investigators repeatedly called the lack of cooperation a distraction for their search.

"Because Misty did not want to do the second polygraph test at the advice of her attorney, because she didn't want to be coerced and cornered and told that she did this, there was a little bit of separation. Some of the family members felt, `OK, she's not cooperating 100 percent.' I hope that turns around now," Brousseau said.

"She's doing it to show that she is cooperating 100 percent so that we can put to rest the idea that there is a family member not cooperating," she added in a phone interview.

The girls were last seen around 12:15 p.m. on July 13, riding their bikes in Evansdale. Their bikes and a purse were later found near Meyers Lake, a popular fishing and recreation area in the northeastern Iowa city of about 4,700 residents. A search involving hundreds of local residents failed to find the girls, and an FBI dive team using sonar equipment concluded last week the girls were not in the lake. Investigators said over the weekend they have reason to believe the girls are alive, and they are treating the case as abduction.

FBI spokeswoman Sandy Breault said Monday that investigators want to interview a person who was paddleboating on the lake around the time the girls disappeared. She said that person, who has not voluntarily come forward, could help investigators learn what happened to the girls but was not considered a suspect.

"They believe that person has information," she said. "They really do need to talk with the person."

Black Hawk County Sheriff's Capt. Rick Abben said last week that investigators were examining "every aspect" of the criminal histories of Cook-Morrissey and her estranged husband, Dan Morrissey, so that they do not overlook any possible leads. He said they were not suspects in the disappearance. But he acknowledged authorities sought a judge's order last week requiring Morrissey to submit to supervision by parole agents while he awaits trial on charges of making and dealing methamphetamine that carry the prospect of decades in prison.

Brousseau said Morrissey also went through a second polygraph test over the weekend after initially declining to do so. Brousseau said the couple had been advised earlier to stop cooperating with police after consulting with Waterloo attorney Tom Frerichs. He said Monday he was not representing the couple but could not comment on whether he'd had any discussions with them.

Brousseau said Cook-Morrissey agreed to Monday's test on the condition that a family member _ she picked Morrissey _ be present in the room. The FBI also promised that she would not face any accusatory interrogation, Brousseau added.

Breault and Abben didn't immediately return phone messages seeking comment on the polygraph tests.