The crash occurred around 9:30 p.m. Wednesday about a mile from Lanai Airport in the Miki Basin area, Maui County spokesman Rod Antone said.
The plane, a twin-engine Piper PA31, burned upon impact, according to Federal Aviation Administration spokesman Ian Gregor. The tail number is listed as N483VA.
Authorities said that they believe everyone aboard the aircraft has been accounted for.
Maui County identified the dead as the pilot of the plane and two Department of Planning employees. Two other planning employees were in critical condition and the deputy attorney for Corporation Counsel, the agency that provides legal services for the county, was in serious condition Thursday morning, according to a nursing supervisor at Queen's Medical Center and the county. All three survivors were airlifted to the Honolulu hospital. No names were released.
Lanai is part of Maui County, and it's common for county officials to travel frequently between the islands via plane or ferry.
The group had been on Lanai earlier that evening to staff a Lanai Planning Commission meeting and chartered a return flight on Maui Air, the county said.
"The man who called 911, the deputy attorney for Corporation Counsel, said he had pulled two of his co-workers away from the fire as best he could because they could not move on their own," the county said in a news release. "All three survivors suffered burn injuries."
At a news conference Thursday, Maui Mayor Alan Arakawa choked back tears, calling the crash a tragedy.
"We're a very small community. Everybody knows each other. Everybody works with each other." he said.
Fire Services Chief Lee Mainaga said the "aircraft down" call came at 9:25 a.m. At first, fire crews didn't see anything when they got to the airport. Then they located the aircraft about a mile south of the airport, in former pineapple land with grass about two-feet high, he said.
Emergency responders received help accessing the grassy area from workers of a general contracting company, Goodfellows Bros., Inc., the company said in a statement.
The plane was "fully engulfed," Mainaga said.
Firefighters extinguished the fire while police tended to the victims.
Lanai City resident Russell deJutley, 20, said he drove to within about 600 feet of the site when he heard about the crash. "I noticed there was smoke coming from the airplane," he said. The area was difficult to find - about a mile from a paved road, he said.
Police spokesman Lt. William Juan said names of the victims wouldn't be immediately released.
State Sen. J. Kalani English, who represents Lanai, said the crash and a December ocean crash-landing off Molokai highlights how often government officials need to fly. Hawaii Health Director Loretta Fuddy was a passenger on the Dec. 11 flight that took off from Molokai's Kalaupapa peninsula. Maui police said autopsy results show she died of cardiac arrhythmia, which was the result of stress from crashing into the water after the single-engine plane lost power.
"These flights are very familiar to me," said English, who flies frequently between Oahu, Maui, Molokai and Lanai. Inter-island travel is especially common for officials in Maui County, which includes Molokai and Lanai.
"It's one of the hazards of our topography, being a multi-island county and district," he said. "Hawaii has very, very safe airports and very, very safe flight routes. For us, I mean government officials, because we have to fly so much to these areas, the probability goes up."
According to an agenda, the meeting at the Lanai Senior Center discussed Lanai's community plan. English said the two staffers who died were women and the three survivors are men.
"This was routine for them," he said of traveling between Maui and Lanai. "At least twice a month they're having to fly out."
Maui Deputy Planning Director Michele McLean said the team had been traveling to Lanai on a regular basis for many months, sometimes by ferry, sometimes by plane.
"We're devastated," she said. "There really isn't anything more to say other than that."
The National Transportation Safety Board would lead the investigation into the cause, with assistance from the FAA, Gregor said.
Billionaire Larry Ellison, the CEO of Oracle Corp., bought 98 percent of Lanai in 2012. He later told financial news channel CNBC he envisions Lanai becoming a "little laboratory" for experimenting with more environmentally sound ways to live.
Lanai, Hawaii's smallest publicly accessible inhabited island, is known as the "pineapple island" because the majority of it was once owned by James Dole of Dole Food Co. Inc., who bought it in 1922. There are some 3,200 residents of the rural island that has one school, one hospital and no traffic lights. It's a 25-minute plane ride from Honolulu.
Lanai resident Robin Kaye, who attended Wednesday's meeting, said it focused on utilities and infrastructure plans. "It was a full house," he said. "There were a lot of people from the community there." The community has been interested in what Ellison's plans for the island are.
Lanai Airport is located three miles southwest of Lanai City. According to the state Department of Transportation, the airport has a single runway and primarily serves scheduled interisland and commuter flights. There are no aviation services listed for the airport and it's not know whether the flight took on fuel or got any service work done while it was there.
Officials for Maui Air couldn't immediately be reached for comment.