Interval has been booked for one count of first-degree murder. Mustafa's body has yet to be located.
Christine Mustafa was last seen by a coworker as she was leaving work on May 10. Mustafa's sister was the last person to hear from her, later that day.
According to police, on Thursday, Mustafa did not arrive for her scheduled work shift, prompting family members to call police for a welfare check.
"She wouldn't just leave her 8-month old child and disappear or vanish like that," said Mustafa's sister, Linda Donaldson.
Phoenix police say they went to her house near Cave Creek Road and Deer Valley Drive the next day and her boyfriend said she walked to work that morning.
Family and friends don’t believe Mustafa would just disappear without her kids, her car, phone and wallet.
Phoenix police released the following timeline:
May 10th, 2017 - Christine was last seen by a coworker as she was leaving work.
May 10th, 2017 - Later in the day, Christine spoke to her sister by telephone. This was the last time Christine had been heard from.
May 11th, 2017 - Christine failed to show for her scheduled work shift.
May 11th, 2017 - The family of Christine called Phoenix Police and asked them to check the on the welfare of Christine. The police went to her home but there was no answer at the door. Christine's boyfriend arrived while police were at the home and he told them Christine had left for work earlier that morning on foot. Phoenix Police entered a Missing Person report at this time.
May 16th, 2017 - Detectives from the Phoenix Police Department served a search warrant on Christine's home in an effort to locate clues to her disappearance.
June 8th, 2017- Mustafa's boyfriend arrested, booked for first-degree murder
On Thursday Erin Neathery, Mustafa's friend and a spokesperson for the family, held a press conference just hours after Phoenix police announced Interval's arrest.
According to Neathery, Mustafa was planning to leave Interval.
"He was consistently accusing her of cheating on him and constantly pushing at her," Neathery said. "So yes, they [her family] had hoped she would get out. Again, this is a common story for women trapped in relationships for whatever reason and sadly she didn't make it."
Neathery described Robert as being charismatic and outgoing — but she also said that he had a "dark side."
"He is quick to snap temper-wise," Neathery said. "I've seen him a couple of times; incidents that just flare up almost instantly — it was a bit of a red flag for me."
Neathery revealed that if Mustafa is found dead, this is the fourth daughter that her mother will have to bury.
"We need closure," Neathery said. "Christine was a lovely girl...very sweet; always ready to help someone. At this stage, the best we can hope for is finding her remains and bringing her home to give her a decent burial."
Diane Strand, a neighbor of Mustafa, said the community is putting together another search slated for Sunday.
"We have a very large area that we are going to search...two miles," Strand said. "I've got people with drones, bicycles and horses...we're still asking for assistance to come out and help search."