Two more mountaineers have died in a fall on Alaska's Mount McKinley, the latest in a string of recent deadly climbing incidents in Denali National Park.

The two climbers were roped to two others, who were critically injured in the late Wednesday fall on North America's highest mountain. Mountaineers at a camp at 17,200-feet witnessed the accident at about 11 p.m. near 18,000 feet.

The climbers fell about 1,000 feet while descending from the deceptively dangerous Denali Pass to the high camp, National Park Service spokeswoman Kris Fister said. The path, used by most climbers to reach the 20,320-foot summit, has a slope of up to 45 degrees. Conditions this year are not unprecedented, Fister said, but are above average in difficulty due to the hard-packed, wind-scoured conditions of the snow.

"There's not a whole lot to grip into," she said.

The park service counsels climbers to use ropes and snow anchors, t-shape pieces of aluminum 24 to 36 inches long that can be buried or pounded into hard snow. It was too early Thursday to know if the climbers used snow anchors, Fister said

Air National Guard pararescuemen from the 212th Rescue Squadron responded to the climbers and confirmed that two were killed. They placed the other two climbers in rescue litters and lowered them to the high camp for emergency medical treatment.

One of the injured was conscious and stable with a broken leg and head injury.

The park service said the second patient was "nonresponsive" with labored breathing. Air National Guard medics worked throughout the night to maintain the patient's airway.

Denali National Park's high altitude helicopter flew to the 17,200-foot high camp at 4:15 am Thursday. The injured climbers were evacuated separately. They were flown to the 7,200-foot Kahiltna base camp, where LifeMed air ambulances flew them off the mountain.

Weather at the time of the accident was clear with relatively calm winds, the park service said.

The climbers were the third and fourth to die this year on Mount McKinley. Seven climbers have died in Denali National Park.

A 76-year-old Italian climber, Luciano Colombo, fell to his death in the same area May 16. Rangers said afterward he had not used ropes and snow anchors while descending from Denali Pass.

Of the 112 fatalities on Mount McKinley since 1932, Fister said, 14 have been at Denali Pass.

The deadly climbing season in Denali National Park began April 28 when falling ice killed 39-year-old Chris Lackey of Houston near the Ruth Gorge. He was in one of two parties camped overnight on the "Root Canal," a glacier landing strip and camping area the 10,300-foot peak known as Moose's Tooth.

On May 12, a 38-year-old member of a four-person climbing team involved in a fall, Beat Niederer of St. Gallen, Switzerland, was found dead from unknown causes near 18,000 feet. His body showed no visible signs of trauma and he may have died of exposure. Colombo fell to his death four days later.

Two climbers who had recently climbed McKinley were killed in the last week on nearby Mount Frances, a 10,450-foot peak.

Jiro Kurihara, 33, of Canmore, Alberta, and Junya Shiraishi, 28, of Sapporo, Japan, were attempting a new route on the west face of Mount Frances.

They were last seen Saturday at a base camp on McKinley. When they did not return, a search was launched Monday. Rangers on a helicopter spotted a body in avalanche debris at the base of the mountain. Their bodies were recovered Wednesday.

The deadliest year at Denali National Park was 1992, when 13 climbers died, including 11 on Mount McKinley and two on Mount Foraker. Two years later, three climbers died on McKinley, two on Mount Wake and two on Mount Hunter.

The deaths led the park service to institute preregistration requirements and special use fees for climbing McKinley and Foraker, which had seen a higher percentage of accidents for the relatively few people who climbed it.

"It provided us the opportunity to put the mountaineering information not only into English, but translated into other languages, basically ramp up our educational program to make people realize what they were getting themselves in for," Fister said.

Climbers had underestimated the risk of the Alaska mountains, she said, especially McKinley.

"People had climbed higher things," she said. "They'd been in the Himalayas, They'd been in the Andes. But McKinley can be far more difficult because of its far-north location, and it's location in the Alaska Range, with the weather systems coming between the north and the south, and sort of smacking together right there in the range."