HANOVER, N.H. – The teens accused of butchering two Dartmouth College professors talked their way into the couple’s home by claiming to be doing a survey and killed them in a plot to steal their ATM cards, prosecutors charged Tuesday.

The allegations – contained in a newly unsealed indictment – mark the first time authorities have given a detailed motive for the slayings last year of Half Zantop and his wife, Susanne.

In the six months leading to the killings, Robert Tulloch and James Parker had gone to four other randomly chosen homes, intending to kill the occupants for their ATM cards, but no one was home or the people who answered the door would not let them in, according to the indictment.

Then on Jan. 27, 2001, they got inside the Zantop home by telling Half Zantop, a professor of Earth sciences, they were students conducting an environmental survey, prosecutors said. The Zantops’ slashed and stabbed bodies were found in their study that evening.

Tulloch, 18, has indicated he will use an insanity defense at his murder trial, which is scheduled to start in April. If convicted, he faces a mandatory sentence of life in prison without parole.

Parker, 17, has pleaded guilty to reduced charges and agreed to testify against Tulloch. The indictment does not say whether the newly released details came from Parker.

Prosecutor Kelly Ayotte would not say if the teens got the Zantops’ ATM cards. However, the indictment said that after the killings, they burned Half Zantop’s wallet along with bloody clothes.

The teens fled after being linked to the crime by the 121/2-inch commando knives they bought on the Internet. They were arrested a year ago Tuesday while hitchhiking in Indiana.

In the months that followed, townspeople and the media had offered a host of theories – a thrill-killing, revenge for some kind of slight, even neo-Nazi hatred. The Zantops, ages 62 and 55, were from Germany and felt their native country had not done enough to atone for the Holocaust.

In an indictment released in December, prosecutors said the slayings took place during a robbery. But they gave no details until Tuesday.

A call to Tulloch’s lawyer was not immediately returned.