New book on the Idaho murders | What sources say happened that night and what it means for the case

MOSCOW, Idaho -- A new book is taking a look at the University of Idaho murder case and who may have been the target of the killings. The book "When the Night Comes Falling" by author Howard Blum, also details what the surviving roommates were said to be doing the night of the brutal attack.

"The lucky break I had in this story was I got out there early," Blum said. "I went out before there was a suspect and so I was able to talk with many people in law enforcement while they were in the hunt."

The book shares the months leading up to the murders of Xana Kernodle, her boyfriend Ethan Chapin, Madison Mogen and Kaylee Goncalves, as well as the horrifying night in November 2022.

RELATED: Idaho college murders timeline: The complete timelines of events

In his reporting, Blum comes to the conclusion that Mogen was the target.

"The authorities, in their mind, are convinced that the suspect came in through the sliding door on the second floor of the house. Once you enter the kitchen area, there's a bedroom that Dylan Mortenson was in on the left-hand side. On the right-hand side was Xana's bedroom. If he was just on a killing spree, it would have been natural - instinctive -- to go to one of those doors. Instead, he goes up this narrow staircase and he turns directly into Maddie's room. I think Maddie was his target."

Sources tell both Blum and ABC News that the two surviving roommates were using their cell phones to communicate during and after the murders.

"According to testimony that was made to the grand jury, the two girls were communicating and they were texting one another not just after, but at one point, during the killings," Blum said.

Blum also wrote about how members of suspect Bryan Kohberger's own family were concerned by his behavior leading up to the arrest.

Sources close to the family told Blum that Kohberger's father, Michael, was on edge as he picked his son up from school that winter.

"He's been reading the headlines. He knows that four students were killed just 12 miles from his son's house. He knows what a troubled son he has," Blum said. "The Moscow police issue a BOLO (be on the lookout), a stop order on a car that is a white Hyundai Elantra. He realizes my son drives a 2015 Hyundai Elantra."

RELATED: The many pre-trial hearings in Idaho quadruple murder case against Bryan Kohberger

According to Blum's sources, Kohberger's own sister later approached their dad, voicing her suspicions about Bryan.

"She goes to Michael and she says, 'Dad, in effect, we have a real problem. I'm starting to believe that Bryan could be involved in these killings.' And what does Michael do? He can't confront it. He just turns around and walks away," Blum said.

Shortly after that, on December 30, 2022, Bryan was arrested at his parents' home in the Pennsylvania Poconos.

The Kohberger family has not commented on the claims in Blum's book.

ALSO SEE Kaylee Goncalves' parents share new details about how daughter killed in Idaho murders was found

Blum also detailed Kaylee's father's tireless pursuit of answers. However, the Goncalves family told ABC News that they "have never spoken with Mr. Blum about the contents of his book."

"When the Night Comes Falling" is out on Tuesday.