Federal wildlife officials raised a formal alarm on Thursday over the deaths of scores of bottlenose dolphins in waters off the east coast, saying that a fast-spreading infection could be attacking dolphin populations from New York to Virginia.

At least 124 of the mammals have washed onto beaches since July, all of them dead or dying, a spokeswoman for the National Marine Fisheries Service said in a conference call with journalists. In July alone, 89 dolphins were beached, seven times the usual number.

The agency, which is responsible for monitoring and protecting marine mammals, declared the deaths to be an “unusual mortality event,” opening the way for federal help in finding the cause.

Experts said anyone who finds a stranded dolphin should not touch it, should keep pets away and should alert the authorities.