A group of scientists and environmental groups has asked the federal government to protect the once-common little brown bat because it is being threatened by a disease that has killed a million bats in the Northeast.

A formal request was filed Thursday with the U.S. Fish and Wildlife Service asking if the bats, once the most common bat species in the Northeast, need protection under the Endangered Species Act because of a fast-spreading, lethal disease called white nose syndrome.

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Thomas Kunz, a bat expert at Boston University who made the request with three environmental groups, said white nose syndrome could cause the little brown bat species to become extinct in North America within two decades.

White nose syndrome was first identified in upstate New York in 2006. Since then it has spread throughout the region.

The bat die-off has caused significant concern among biologists and conservation groups, not only because of potential extinction of one or more species but also because the night-flying mammals play a critical role in keeping insect populations in check.