CAMBRIDGE, Mass —  Three of the victims of a plane crash in the mountains of Afghanistan were American women who had been working to help improve the struggling nation's public health system.

Cristin "Cristi" Gadue, 26, a native of Burlington, Vt., Amy Lynn Niebling, 29, of Somerville, and Carmen Urdaneta, 32, of Brookline, worked for Cambridge-based Management Sciences for Health (search).

"Cristi, Amy and Carmen were vibrant committed young women doing great work," said Jonathan Quick, president and CEO of Management Sciences for Health. "They stood out among the people who worked with them as women of tremendous energy, of tremendous commitment, and women who cared a lot about people less fortunate."

Six Americans were believed to have been on board, the U.S. Embassy in Kabul (search) said, double the number previously reported.

The Kam Air (search) Boeing 737 disappeared Thursday after it was unable to land at Kabul because of poor weather. The plane's wreckage was spotted Saturday in the mountains east of Kabul, and officials said there appeared to be no survivors.

Gadue had been working in Kabul since September 2003. Niebling and Urdaneta were on a three-week visit to Afghanistan and had been scheduled to return to Cambridge this weekend.

"To have women who are that passionate about their work is inspirational," said Stacey Irwin Downey, a senior program officer at the company.

Niebling, originally from Omaha, Neb., was a newlywed on her first field assignment for MSH. Urdaneta, a native of Topeka, Kan., had been with the company for the five years, and had worked to combat AIDS in Africa and to improve health services in Angola. Urdaneta was well known in the global health community for her writing and photography.

Friends said it had been obvious even in high school that Gadue would do something selfless.

"She had a quiet demeanor, but her heart was huge," said Caitlin Falzone, who grew up two doors down from Gadue and now lives in Tempe, Ariz. "She was always out to help people."

Gadue's father said his only daughter had lived a full life.

"All I can say is the life of my daughter was fully complete," said Mike Gadue, who lives in Burlington, Vt. "Not to say I wouldn't want it much more complete by it being extended, but she's a good example of what can be done. I'm very proud of her.

"For a father to be able to say he has no regrets, that's a big thing," Mike Gadue said. "An important thing."