NTSB and FAA Launching Investigation After Idaho Lake Plane Crash Kills Golf Pro, Kids

Officials are looking into what caused two planes to collide over a lake in Idaho, ultimately killing eight people, including PGA golf pro Sean Fredrickson and his three children.

In a statement to PEOPLE, Federal Aviation Administration (FAA) spokesman Ian Gregor confirms that both the FAA and the National Transportation Safety Board (NTSB) will investigate the fatal crash, which unfolded on Sunday afternoon above Coeur d’Alene Lake near Powderhorn Bay.

Gregor notes that the NTSB is the lead investigative agency that will determine the "probable cause" of the accident, which typically takes "a year or more to determine."

An NTSB representative told PEOPLE that two of their investigators arrived at the crash scene on Monday and interviewed witnesses and first responders. They also examined the recovered wreckage, though most of the debris is still underwater.

"We're still in the very early, early stages, and it's an ongoing investigation," the NTSB spokesperson told PEOPLE.

Kootenai County Sheriff's Office Lt. Ryan Higgins also noted during a press conference on Monday that the NTSB was investigating all of the debris that was recovered at the site and working with the insurance companies to help recover the planes, which he predicted could take "several days to a week."

RELATED: PGA Pro Sean Fredrickson and His 3 Kids Among Passengers Killed in Plane Crash Over Idaho Lake

According to the Kootenai County Sheriff's Office, two planes crashed into each other above the lake around 2:20 p.m. on Sunday. Multiple units and the U.S. Coast Guard responded to the scene, though the sheriff’s office’s sonar team eventually located both planes at about 127 feet and recovered two victims' bodies from the water.

A third victim was later recovered by dive teams and the location of two additional victims had been determined on Monday, with technical divers expected to begin recovery efforts for their bodies, Higgins announced during the press conference.

One of the aircraft involved was a charter plane operated by Brooks Seaplane and a pilot and five passengers — four of whom were related — were aboard, while the other plane was a Cessna 206G registered to Lewiston, Idaho, Higgins said.

On Tuesday, he verified that there was a total of eight passengers and crew involved, but did not formally release the victims' identities, calling the crash "the most tragic incident" he's seen in his 20 years of law enforcement.

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In the meantime, family members of Fredrickson confirmed that the victims included the Oregon father, his son Hayden Fredrickson and his stepchildren Sofia and Quinn Olsen.

April Fredrickson, Sean's wife and Sofia and Quinn's mother, told Fox affiliate KPTV that her family had "died while they were on an adventure," and expressed her heartbreak over the tragedy.

"Everyone was so excited to be on that plane… I think that, at the end of the day, they died doing what they loved which was, you know, being together," she told KPTV. "I don’t know a person that did not like my husband. He just had an infectious personality. Always wanted to take care of everybody else."

“My daughter was 15 and … she loved life, she literally loved life. My little guy was Quinn. He was 11 and he was my best friend. He was just always in tune with what I needed," April continued of her kids. "My stepson [Hayden] had just gotten his license, and he had his car, and he had a girlfriend and his life was good. His life was so incredibly good, and he was a happy kid."

RELATED: 8 People Feared Dead After 2 Planes Collide in 'Terrifying' Crash Over Idaho Lake

Along with his family members, Sean's employers and colleagues confirmed his death and expressed their condolences over the heartbreaking incident.

The Professional Golfers' Association of America (PGA), where Sean worked as the president of the Pacific Northwest Section, issued a statement on their website and remembered him for his love of mentoring others.

"Sean was an incredible father, husband, PGA Professional, mentor, and leader," the organization wrote. "There are never words to communicate the pain of such a loss."

"We are all better because of Sean’s leadership over the past twelve years," the statement continued. "His passion to help many of you was driven by his genuine desire to serve his fellow PGA Professionals. You will carry on this passion. As you remember Sean, reflect on the enthusiasm, the integrity, the passion, and the love we saw in him every day. We will miss you, our friend."