Newly released details about a 2019 plane crash that killed nine of 12 members of an Idaho family revealed that a buildup of snow and ice on the wings and other parts of the plane played a significant role in the crash shortly after takeoff.

A number of documents made public by the National Transportation Safety Board concluded that the pilot did not completely remove snow and ice before taking off during a storm, that more people were on board than available seats, and that the plane was more than 100 pounds over its maximum weight limit.

An investigation of the crash is ongoing, and a final analysis will be released at a later date, a safety board spokesman said on Monday.

The plane, a Pilatus PC-12 single-engine turboprop, was traveling from Chamberlain, S.D., about 140 miles west of Sioux Falls, S.D., to the Idaho Falls Regional Airport, when it crashed before noon local time on Nov. 30, 2019, according to the N.T.S.B.

Nine people, including two children, died, spanning four generations of the family. The victims included Jim Hansen and Kirk Hansen, brothers and founders of Kyäni, a health and wellness company; their father, James; Kirk Hansen’s sons Stockton and Logan; and Kyle Taylor and Tyson Dennert, his sons-in-law.

Three people survived the crash, but they were seriously injured, including Kirk Hansen’s son Josh, Matt Hansen and Thomas Long.

According to a witness statement from Carey Story, the owner of the lodge where the family was staying, the pilot thought there would be favorable weather for flying that day between 11:30 a.m. and 2:30 p.m. when the temperature would be just above freezing at 33 to 34 degrees. The safety board has not identified the pilot.

The documents include a weather report, which noted that on the morning of the crash the National Weather Service in Sioux Falls reported conditions “about as messy and complex as a forecast gets” with snow, ice and freezing rain possible.

The pilot and a passenger worked for about three hours to remove snow and ice that built up on the plane overnight, according to the witness statement. The pilot and the passenger used a 7-foot ladder to remove snow and ice from the plane, but it was not high enough to reach the top of the plane’s tail. “It was snowing hard at the time the pilot took off,” Mr. Story said in his statement, and he said he asked the pilot to stay another night at the lodge. The pilot said they needed to return home.

Mr. Story said he saw the pilot complete a flight control check before takeoff, and that all flight controls appeared to move freely. He said the pilot told him the plane was “98 percent good and the remaining ice would come off during takeoff.”

When the pilot was removing ice off the plane that morning, he commented that “it’s coming off pretty good,” according to a separate witness statement from Dusten Hrabe, the Chamberlain Airport manager. The weather seemed to be deteriorating, Mr. Hrabe said in his witness statement.

A man who was near the airport before the crash, Scott Lewis, said he was sweeping snow off his daughter’s car before the plane took off, according to an eyewitness report he sent to the N.T.S.B.

“It was snowing so hard I could not see the plane,” he said in an email to the safety board.

The newly released documents also show that there were more people on the plane than available 10 passenger seats. Two people were likely sitting in the aisle of the cabin, according to the documents.

The Sioux Falls Argus Leader reported on the newly released documents and video last week, calling the crash the deadliest in the state since 1968.

In a preliminary report released about a month after the crash, the board said that two of the plane’s warning systems — the cockpit stall warning and the stick shaker — were activated about one second after liftoff. Another warning system, the stick pusher, was activated after about 15 seconds after liftoff.

The plane climbed to 460 feet in the air before it crashed in a cornfield less than a mile from the airport, the board said. Radio communications were not received from the pilot, and the plane never established radar contact, according to the agency.