From left: Steven Gibson Jr., mom Michelle Gibson and friend Erik McBee
Maricopa Police Dept.
04/03/2013 at 06:10 PM EDT
In a frantic 911 call on March 1, Michelle Gibson tearfully told police about a grisly discovery she made at 2 a.m. in the family's Peoria, Ariz., home.
"There's blood everywhere!" Gibson cried. "I'm with my kids and I just got home and my husband's out in the garage dead!"
After a month-long investigation, detectives now say it was all a sinister act
. Authorities allege that Steven Gibson, 39, the owner of a machine shop, was the target of a family murder plot hatched by his wife and their son Steven Jr., 15, who took part in stabbing him.
"This is a complete shock to the rest of the family," Amber McGonagill, 23, Steven Gibson's stepdaughter, tells PEOPLE. "I have no clue why my mom would do this. My dad was a loving man and was not abusive in any way."
Michelle Gibson, 41, a nursing school student, allegedly offered $1,000 each to two of her son's friends on Feb. 28, telling them "they needed to kill [her husband] before he killed her or [his son]," according to court documents filed by prosecutors.
The plan was to use chloroform to knock Steven Gibson out, drive him to a nearby park, shoot him in the head, and make it look like a drug deal gone bad, the documents say.
The mom also allegedly told the two boys that they needed to make sure her husband's body was found "so a money inheritance could be paid out."
Later that evening, one of the two boys, Erik McBee, 16, who worked at Popeye's Chicken, allegedly beat a sleeping Steven Gibson in the head with a baseball bat, according to the prosecutors' complaint.
Steven Jr. then later "drove [a] large knife he was holding into his father's chest at least three times," the documents say. "[McBee] then took the knife and slit the victim's throat."
Michelle Gibson claims she was out shopping with her kids at the time, but her 16-year-old daughter Alysa told police that after the three of them drove to a store, Steven Jr. was driven back to the house by his mom after he received a phone call.
Later, after they picked him up in a church parking lot, he allegedly stated: "What's done is done," prosecutors allege.
The scheme fell apart when McBee, who was helping Steven Jr. move the body, heard police sirens in the distance, fled the scene but later turned himself in to police at the house, documents say.
Michelle Gibson, Steven Jr. and McBee have each been charged with first-degree murder and remain in police custody. Attempts to reach their attorneys were unsuccessful.
Police have not yet offered a possible motive. The Gibson household has a record of domestic violence calls between the parents, but they consistently refused to press charges, Peoria Police Department spokeswoman Amanda Jacinto tells PEOPLE.
The elder Steven Gibson had been arrested in Nov. 2012 for allegedly assaulting an officer during a DUI stop, but none of the family members had any other significant criminal history.
Asked if the Gibsons' 16-year-old daughter would be implicated in the plot, Jacinto added, "More arrests could occur depending on how the investigation plays out. This is a complex case and there's still a lot of evidence to review."