HAVERHILL, N.H. – One of two Vermont teen-agers accused of stabbing two Dartmouth College professors to death was indicted Thursday on first-degree murder charges.

A grand jury charged Robert Tulloch, 17, with two counts of murder in the Jan. 27 deaths of Half and Susanne Zantop in their Hanover home. An arraignment date was not set.

Prosecutors are trying to certify his co-defendant, James Parker, 16, as an adult before seeking an indictment against him.

“In my experience, it could be a number of months before that takes place,” prosecutor Kelly Ayotte said.

Investigators said they were unsure of a motive for the killings or a possible connection between the German-born professors and the Chelsea, Vt., teens.

Court documents say police found two military-style knives in Tulloch’s bedroom with Susanne Zantop’s blood on them. They say Parker bought similar knives on the Internet several weeks before the murders, a purchase that led investigators to fingerprint the teens.

Bloody footprints found at the scene also link Tulloch to the crime, according to the documents. A fingerprint found on a knife sheath left at the scene matched Parker’s, the documents say.

Prosecutors are seeking blood and handwriting samples from Tulloch to match with evidence found at the scene and in his home.

Tulloch and Parker left home in the days after the murders, telling friends they were going rock-climbing in Colorado.

They turned back because Tulloch had a gash on his knee that became infected. He said he injured himself while walking through woods.

On Feb. 15, the teen suspects voluntarily gave fingerprint impressions to police. They fled hours later, and were arrested on Feb. 19 at a truck stop in Indiana after a nationwide manhunt.

Susanne Zantop, 55, was chairwoman of the German studies department at Dartmouth. Half Zantop, 62, taught earth science.