Authorities declined to name the victims at the farm, saying they were still notifying family members. A nurse who answered at the hospital late Monday would not release any information about the woman's injuries.
Don Simonsen of the National Weather Service in Glasgow said the tornado touched down west of Reserve between 7:15 p.m and 7:45 p.m. before crossing into North Dakota, where it weakened.
There also was at least one tornado reported about 20 miles south of Flaxville in Daniels County, Simonsen said.
Before the tornado touched down, authorities reported heavy hail and rain, along with strong wind pushing through.
Medicine Lake resident Brandon French said he and five other men were watching the tornado from a hillside when they got a call that a farm to the west had been hit. The men got into a pickup truck and were among the first to arrive.
The house, barn, lean-to and various buildings that had been there just moments before were simply gone, he said.
"There's no houses, there's no buildings, there's nothing left," French said. "The vehicles were all turned over, there was a pickup plowed into a tractor -- it just wrapped the pickup around the tractor. You couldn't peel it off."
Ulrickson said the damage was devastating -- the house was completely gone from the foundation. The tornado also destroyed a mobile home and other buildings, he said.
"We had a Quonset hut that was crushed like a pop can," he said.
Northeastern Montana is part of the Hi-Line, with vast stretches of plains and rolling wheat fields just south of the Canadian border. The area where the tornado touched down was "extremely isolated," Fulkerson said.
Sheridan County's population has been shrinking, with just over 3,200 people in 2008, according to the U.S. Census Bureau. The main source of income for its residents is farming.
In the northwestern corner of North Dakota, a sheriff's dispatcher in Divide County said they were no reports of storm damage or injuries.
Tornadoes are relatively rare in Montana, although on June 20, a tornado tore apart the state's largest indoor arena.
The June tornado touched down on top of the Rimrock Auto Arena in Billings, tearing off the roof and some siding and causing extensive damage to the interior. The arena was not in use at the time.
The tornado was the first large tornado to hit Billings in more than a half-century.
City officials said hundreds of households also suffered some damage, either from severe winds or hail that accompanied the twister.
James Kraft, Yellowstone County's emergency coordinator, has said insurance is expected to cover most or all of the rebuilding costs caused by that tornado.