UPDATE 2:25 p.m.
Authorities say the falling ice killed at least six and injured eight.
There could be about 10 people missing, Civil Protection official Gianpaolo Bottacin was quoted as saying by the online version of Italian daily Corriere della Sera. But Bottacin later told state television that it wasn't yet possible to provide a firm number.
Rescuers were checking license plates in the parking lot as part of checks to determine how many people might be unaccounted for, a process that could take hours.
ORIGINAL 8:10 a.m.
A large chunk of Alpine glacier broke loose Sunday afternoon and slid down a mountainside, slamming into more than a dozen hikers on a popular trail on the peak. At least four people were killed, Italy's state television said, and seven others were injured.
RAI state television didn't cite a source for its report of fatalities. But earlier the National Alpine and Cave Rescue Corps tweeted that the search of the involved area of Marmolada peak involved at least five helicopters and rescue dogs. Separately, emergency dispatchers in northeast Italy tweeted that about 15 hikers were believed to have been in the stricken area.
The segment that broke away is known as a serac, or pinnacle of a glacier.
Marmolada, towering about 3,300 meters, is the highest peak in the eastern Dolomites.
"A breaking away of rock provoked the opening of a crevasse on the glacier, leaving about 15 people involved,'' the emergency dispatchers tweeted.
The Alpine rescue service said in a tweet that the segment broke off near Punta Rocca (Rock Point), “along the itinerary normally used to reach the peak.”
It wasn't immediately clear what caused the section of ice to break away and rush down the peak's slope, but the intense heat wave gripping Italy since late June could be a factor, Walter Milan, an Alpine rescue service spokesperson told RAI state TV.
“The heat is unusual,'' Milan said, noting that temperatures in recent days on the peak had topped 10 C. ”That's extreme heat" for the peak, Milan said. “Clearly it's something abnormal.”
The injured, including a person in critical condition, were taken to several hospitals in the regions of Trentino-Alto Adige and Veneto, according to rescue services.