TOKIO, N.D. -- A federal investigation has been launched into a fire the morning of Friday, May 22, at the Spirit Lake Reservation residence of Erich Longie Jr. and Tammy Longie, the FBI confirmed.

FBI spokesman Kevin Smith said firefighters responded to the blaze around 8 a.m. Friday. Smith said the house, which was unoccupied at the time, is considered a total loss, although Bureau of Alcohol, Tobacco, Firearms and Explosives (ATF) spokesperson Ashlee Sherrill said it's too early to make any official determination.

ATF is leading the investigation into the fire, along with the Bureau of Indian Affairs, Smith said. Sherrill said ATF has sent certified fire investigator agents to the scene, but information about the blaze remains limited. She estimated it will take investigators about a week to determine the cause of the fire.

The Longies are incarcerated on four federal assault charges, and are accused of assaulting four children who were in their care, including 5-year-old Raven Thompson, who died of her injuries on May 5.

U.S. Attorney for North Dakota Drew Wrigley confirmed that his office is considering the possibility of a connection between Friday's fire and the death of Raven Thompson, although he emphasized that the cause of the fire remains unknown.

"It's possible there is a connection in the current and ongoing investigation regarding Raven's death and Zane's injuries, as well as the injuries to the two other children who were named in the criminal complaint," Wrigley said. "But again, it's too early to assess that. The investigation is federal, and our current investigation is just, if anything, widened to also include this."

The Longies appeared for a preliminary and detention hearing Thursday afternoon, May 21, at which FBI Special Agent Jerrod Birchler testified that there had been a number of threats made against the Longies on social media. While the Longies themselves had not reported the threats, Birchler said they had been "fearful" enough to leave their home to stay with family nearby, and Tammy Longie had attempted to obtain a firearm for protection.

The Longies' respective attorneys argued in favor of electronic home monitoring while the pair await trial, but U.S. Magistrate Judge Alice Senechal, taking into account the threats made against the Longies, ordered them to remain incarcerated. Christopher Lancaster, the public defender representing Tammy Longie, asked Senechal to reconsider her decision, arguing that the Longies should not be penalized for the actions of others, but Senechal let her decision stand.

Smith said he could neither confirm nor deny whether the fire is being treated as suspicious at this time. He did say that it will be investigated in conjunction with the existing FBI investigation.

"We have as many questions as the public does," Smith said. "We're going to get to the bottom of how this plays into the current investigation into the death of Raven Thompson."