A single-engine plane owned by the Civil Air Patrol in Alaska swiped a six-story office building in downtown Anchorage early Tuesday morning, ripping off a wing, then crashed into the next-door building, killing the pilot and sparking a roaring, fuel-fed blaze.

The plane, a Cessna 172, had one occupant, officials said. Authorities identified him as Doug Demarest, a licensed pilot, a first lieutenant with the Civil Air Patrol and an adventurer and photographer who lived in Anchorage. A spokeswoman for the CAP, a federal search and emergency-services agency with more then two dozen aircraft in Alaska, said the flight was not sanctioned but wouldn't say whether the plane was stolen.

The FBI took the unusual step of taking over the crash investigation from the National Transportation Safety Board. Authorities provided few details Tuesday about what they had learned so far, but Staci Feger-Pellessier, a spokeswoman for the FBI in Alaska, said officials didn't believe the crash was an act of terrorism.

NTSB Alaska Region Chief Clint Johnson said the FBI was brought into the investigation because of unspecified evidence investigators uncovered. "Some evidence warrant(ed) us bringing them into the investigation," said Johnson, who declined to elaborate further.

The pilot's wife, Kate Demarest, is an attorney who worked on the sixth floor of the building first struck by the plane, the Brady Building, at 1031 W. Fourth Ave. The crash happened around 6:18 a.m., according to Johnson. She was not at work at the Anchorage office of Dorsey and Whitney, a national law firm. Kate Demarest, an associate at the firm, recently played a role defending the Fairbanks Four, a case involving a group of young men recently released from prison after being convicted of murder more than a decade ago.

"Based on the very limited facts we have, we believe it was a personal tragedy and a tragedy for this family," said Bryn Vaaler, chief marketing officer with Dorsey and Whitney's office in Minneapolis. "She is a valued employee and is obviously involved in a personal tragedy, so that's all I can tell you."

Employees at the Anchorage office were asked to stay home Tuesday, Vaaler said.

A website maintained by Doug Demarest said he was a former National Park Service ranger and Outward Bound instructor as well as a photographer who enjoyed taking pictures of the outdoors and adventure enthusiasts. The website said he and Kate had a child. She declined to be interviewed when approached by a reporter at the family's home Tuesday afternoon.

The photo galleries and other details at www.dougdemarest.com have been taken down, but some material has been preserved by the Wayback Machine, which periodically archives Web pages.

Luc Mehl, an outdoor adventurer and frequent winner of grueling wilderness classics in Alaska, said Doug Demarest was "one of my main paddling partners" and an elite whitewater boater who often participated in summer day trips.

Mehl described Demarest as the best packrafter around, always willing to share his advice on the sport and photography.

A neighbor who lives near the Demarest home in the Geneva Woods neighborhood said he saw three police cars, a fire truck and paramedics at the house late Monday night.

Jennifer Castro, spokeswoman for the Anchorage Police Department, said medics and police responded to a residence in that block just after 10 p.m. Monday night for a medic assist call. She said privacy rules prohibited her from giving the precise address, but the block matches the Locarno Street address where the Demarests live.

A patient was evaluated by medics at the home and no transports were made, according to the police department. No crimes had been committed, Castro said.

Two buildings damaged

The plane first hit the Brady Building about four stories from the ground, then crashed into the nearby Carr-Gottstein Building, where it ignited a fire.

Anchorage police spokeswoman Anita Shell said the plane was completely on fire within minutes.

The Brady Building, at 1031 W. Fourth Ave., is a 90,000-square-foot downtown office building. AFD Assistant Chief Alex Boyd said no one was inside the office area where the plane hit, with damage mostly to the exterior.

The building, along with the Carr Gottstein or "Whale" Building, at 310 K St.. houses offices of the state Law and Corrections departments and the District Attorney's Office. Both offices were closed Tuesday morning, though the Brady Building was reopened around 1 p.m.

The Carr-Gottstein Building was scorched in the post-crash fire, with exposed insulation flapping in the wind Tuesday afternoon.

An inspection by fire officials determined the buildings weren't structurally damaged, Feger-Pellessier said. There were no injuries on the ground, according to Boyd.

The crash caused a power outage downtown. Boyd said the airplane struck an outside electrical transformer as it crashed, leading authorities to shut down power to the area, affecting about 600 customers. Electricity was restored to most within a half hour.

At the time of the crash, Ted Stevens Anchorage International Airport was reporting scattered clouds at 7,000 feet and overcast clouds at 19,000 feet, with "nothing low-level." Winds from the southeast were 20 mph, with gusts to 29 mph.

Merrill Field, the airfield east of downtown Anchorage where the CAP's Polaris Composite Squadron is based, reported few clouds at 7,000 feet and overcast clouds at 20,000 feet, with winds from the south at 9 mph gusting to 21 mph.

Andy Dixon, a forecaster at the National Weather Service's Anchorage office, said observers reported 10 miles of visibility at both airports.

'Not authorized to fly the aircraft'

The CAP is a federally chartered auxiliary of the U.S. Air Force. Its volunteer members conduct search-and-rescue operations as well as disaster relief.

In 2015, the Alaska patrol consisted of 573 senior members and 187 cadets, according to a fact sheet provided by CAP national public affairs Deputy Director Julie DeBardelaben in Alabama. Its fleet had 28 motorized aircraft and three gliders.

The Air Force credited the Alaska patrol for saving one life this year based on a mission, DeBardelaben said. She said the wing was also credited with finding 29 other people in search-and-rescue operations in the state.

At Merrill Field, maintenance crews conducting a routine morning check of hangars at the field between 6 a.m. and 7 a.m. found a hangar operated by the CAP's Polaris Composite Squadron with its main door open, airport manager Paul Bowers said.

"There was no breach in the security fence and there were no signs of forced entry," Bowers said.

The maintenance workers didn't note whether any aircraft were missing, Bowers said, but simply secured the hangar before alerting the squadron's head, Maj. Randy Smith.

"The door was left open, and of course, there's a fairly substantial south wind, and the door was facing directly into it," Bowers said. He likened it to leaving a garage door open at a home.

"Hangar doors are generally not left open because they are generally not empty boxes," Bowers said. "It's unusual to have the hangar doors left open; it's not illegal or immoral, but it's not common."

A statement released by the CAP's national headquarters in Alabama said Doug Demarest joined in 2010, but "was not authorized to fly the aircraft."

CAP became aware of the errant aircraft around 6:45 a.m. -- 20 minutes after the crash -- when the Rescue Coordination Center in Anchorage called to report an emergency beacon was going off on the aircraft, said the DeBardelaben.

Johnson says initial accounts suggest the plane approached Anchorage from Cook Inlet before it crashed.

Plane circled before crash

Eyewitnesses, meanwhile, described the plane as circling before the crash.

"I seen the plane coming in and it did a total complete turn and then boom," said Thomas Connell, who witnessed the crash.

"It flew over us twice and then crashed," Connell said. "It was just way low, and then it started sinking on in."

"I heard it circling and I knew it was having problems," said Mike Coumbe, who lives a few blocks away. "I heard the plane and tried to see the plane and I heard it hit -- it just stopped."

Inside Snow City Cafe across the street, a group of about five employees was beginning work for the day when the plane crashed, according to Vince Maiorano, a cook there.

"We heard a noise -- a loud 'whooshing' noise -- didn't know what it was," Maiorano said.

As the employees stepped outside, the plane -- which had just hit the transformer, knocking out the restaurant's power -- was just beginning to catch fire, said Maiorano, who captured video of the wreckage.

Alaska Dispatch News reporters Annie Zak, Alex DeMarban and Devin Kelly contributed to this story.