HALIFAX -- Investigators believe a young Halifax mother reported missing more than four months ago was murdered the day she disappeared by someone she knew, police said Monday as they announced an arrest in the case.

Catherine (Catie) Miller, 29, was last in contact with her family on July 15 and reported missing a few days later. Her body has not been found.

Halifax police Supt. Jim Perrin of the Criminal Investigation Division said investigators suspected in July that Miller was the victim of a homicide, but only informed the family Sunday that her disappearance was being treated as such. Miller has a young son.

"Our primary function is to find out what happened to Ms. Miller and if we have to hold back information to do that, we will," Perrin told a news conference.

"We can understand the impact that can have on people, particularly her family."

Police have charged Jason James Johnson, 29, with first-degree murder in Miller's death and indecently interfering with a dead human body. Perrin said the second charge refers to "altering or affecting the integrity" of a body, but would not elaborate.

Investigators are actively searching for Miller's remains and Perrin said he's confident they will be found.

Perrin said Johnson and Miller knew each other, but would not disclose the nature of their relationship or say when they met.

He would also not talk about a possible motive or what specifically led police to believe a crime had occurred.

"Shortly after Ms. Miller was reported missing, there were some suspicions that were evident to investigators," he said. "We transitioned the investigation fairly quickly to the homicide unit and it's been a very involved investigation up to this point."

Police said their investigation is ongoing and have asked anyone with information on Miller's disappearance or homicide to come forward. Perrin also did not rule our further arrests in the case.

Miller's family organized a number of public appeals for information since she was last seen, including vigils to highlight their search for her. A Facebook page called Find Catherine "Catie" Miller had more than 2,500 followers Monday.

Johnson was arrested without incident Saturday in Halifax. Perrin said officers met personally with the family Sunday to provide them with an update on the case.

"This is devastating news," he said. "We can only begin to try to imagine what they must be going through at this time."