HAVERHILL, N.H. – Armed with reams of forensic evidence and details about a grisly crime scene, prosecutors will be back before an investigative grand jury on Thursday in the case of the Dartmouth professor murders.

The grand jury also is scheduled to meet Friday, and prosecutors have said they would seek an indictment against the older of the two suspects – Robert Tulloch, 17 – by the end of the month. Tulloch’s sometime girlfriend, Christiana Usenza, is expected to testify Thursday.

The Grafton County Superior Court grand jury met for two days in mid-March. Like all grand juries, its proceedings are secret.

Tulloch and James Parker, 16, both of Chelsea, Vt., are charged with first-degree murder in the Jan. 27 stabbing deaths of Half and Susanne Zantop in their Hanover home. Investigators have not established any connection between the suspects and the victims, or determined a motive, but have said they are considering as possibilities burglary gone awry or thrill-killing.

All grand juries have the power to subpoena witnesses and records and to issue indictments – formal charges. Experts say convening an investigative grand jury lets prosecutors call more witnesses and collect more evidence than they would need just for an indictment.

Parker is a juvenile under New Hampshire law, and prosecutors are seeking to try him as an adult. The certification process could take months and would come before indictment.

Police found two military-style knives with Susanne Zantop’s blood on them in Tulloch’s bedroom, according to documents. The documents say Parker bought similar knives on the Internet several weeks before the murders, purchases that led authorities to question the pair and take their fingerprints.

Investigators have confirmed that in addition to being stabbed repeatedly in the head and chest, each of the victims’ throats had been cut. They said Half Zantop had been stabbed in the heart and had defensive wounds on his hand.