They are pursuing it as a "death investigation," Jake Wark, a spokesman for the Suffolk County District Attorney's office, tells PEOPLE. "The medical examiner has asked for additional specialized testing, toxicology and so forth."
Dailey, 23, from Charlotte, N.C., was last seen at his Boston apartment on Oct. 2 when he ate pizza and watched TV with his roommate, Miles Smith. Smith has said he went to bed and never saw Dailey again.
Smith and Dailey's family have said they noticed nothing unusual about Dailey's behavior in the days prior to his disappearance. But after a frantic week-long search, a rowing coach found Dailey's body in the Charles River on Oct. 9.
Dailey's family is in "shock" over his death, his sister wrote on "Missing Jonathan Dailey," the Facebook page created in her brother's honor.
"We are all in agony, disbelief and shock right now," Robyn Dailey Hoffman wrote Thursday, "not only from the helplessness of having Jon be missing and the overwhelming grief of knowing he has died, but also because of the horror of not knowing what happened.”
She says the family has been given little information about his death and has yet to schedule a funeral.
"The investigators told us it will probably be a long time until they know and we will not be told anything during the process, to keep the investigation as pure as possible," she wrote. "It feels like three separate blows, and each one feels like it's too much to bear ... right now, we need prayers."
Wark tells PEOPLE that while authorities await test results, "We've also got our state police detectives interviewing friends, roommates and going through his bank history. The latter is for two reasons – maybe he bought a cinderblock or chain on that day he disappeared or maybe he's into somebody for some money."
"What we're trying to do is find out as best we can whether this was intentional on his part or something someone else did," he adds. "What we're after is the truth, whatever that truth may be."