By Tim Ghianni

(Reuters) - The wife of a man accused of abducting two Tennessee girls after killing their mother and older sister told authorities that he was obsessed with one of the younger girls and planned the crime for over a year, a state investigator testified on Monday.

The testimony came during a preliminary hearing for Teresa Mayes, who is charged with two counts of first-degree murder and two counts of especially aggravated kidnapping in the high-profile case.

Prosecutors say Teresa Mayes assisted her husband, 35-year-old Adam Mayes, in the kidnappings in late April. Adam Mayes shot himself to death in northern Mississippi on May 10 as police closed in on him.

His mother, Mary Mayes, is also charged with two counts of especially aggravated kidnapping for helping transport the missing girls to a hiding place in the Mississippi woods where they were ultimately found by authorities.

During Monday's hearing, Tennessee Bureau of Investigation Agent Valerie Troutt read a written statement given to investigators by Teresa Mayes in which she said that on the day of the kidnappings, she took the two younger girls, 12-year-old Alexandria Bain and 8-year-old Kyliyah Bain, on a drive.

Teresa Mayes told the investigators that when she returned, she found her husband outside the Bain house with two bodies wrapped in a tarp, Troutt said.

Mayes told the investigators that her husband had poisoned the father, Gary Bain, with a combination of drugs, Troutt said.

Mayes said Adam Mayes lured the mother, Jo Ann Bain, from her home, hit her in the head with a wooden board and strangled her with a rope and smothered the eldest daughter, Adrienne Bain, to death, according to Troutt.

Teresa Mayes described in her statement how she helped bring the bodies and the girls back to Mississippi and hid the girls in the woods, Troutt said.

The bodies of the mother and the older child were discovered May 4 in shallow graves behind a double-wide trailer Adam Mayes had shared with his wife and parents.

Teresa Mayes told authorities her husband already had Social Security numbers and was building new identities for the younger girls, according to testimony from Troutt.

On cross-examination, a defense lawyer questioned Troutt on the circumstances that led to Teresa Mayes giving her statement to authorities and whether she had been properly informed of her rights.

At the conclusion of the day's proceedings, Hardeman County General Sessions Court Judge Chip Cary ordered both Teresa and Mary Mayes' case bound over to a grand jury, which will begin considering the case in January.

(Editing by Philip Barbara)