Three environmental groups and three Chevy Chase residents notified federal agencies Wednesday that they plan to sue over Maryland’s proposed light-rail Purple Line unless the agencies better study the project’s potential impacts on two rare shrimp-like creatures.

A letter to the U.S. Fish and Wildlife Service and the Federal Transit Administration said the agencies had violated the Endangered Species Act by not properly considering a light-rail line’s impacts on the Hay’s spring amphipod, which has been listed as endangered since 1982, and the Kenk’s amphipod, which is a candidate for federal listing. The groups — The Center for Biological Diversity, Friends of the Capital Crescent Trail, and the Center For Sustainable Economy — requested a meeting with the U.S. Fish and Wildlife Service to discuss a more detailed analysis of the potential impacts. The letter also was signed by three Chevy Chase residents: John Fitzgerald, Christine Real de Azua and Deborah Ingram. The line would run behind some backyards in Chevy Chase.

The U.S. Fish and Wildlife Service found in January that construction of a 16-mile light-rail Purple Line between Montgomery and Prince George’s counties would have “no effect” on either animal. The agency said the Hay’s spring amphipod’s habitat in the District’s portion of Rock Creek Park — 4.5 miles downstream of the proposed rail line — would be too far away to be affected. Habitat for the Kenk’s amphipod near the Coquelin Run tributary in Chevy Chase also would not be harmed, the agency said.

However, the groups’ letter said that their own expert, an American University professor, has found that the project’s potential impact on the two creatures “warrants further scrutiny.” The professor, David Culver, found seven freshwater springs and seeps and two small wetlands where the creatures could live near the proposed rail line’s path, the letter said. The groups also said the 24 acres of trees that would be cut for the line and the additional impervious surfaces it would bring would increase stormwater runoff into nearby streams, further degrading them.

The groups have about 60 days to file a lawsuit.

A Purple Line would run east-west, mostly along local streets, between New Carrollton and Bethesda. It would connect Metrorail lines with Amtrak and MARC commuter rail stations.

Maryland officials have said they hope to begin construction in 2015 and open it to service in 2020. State officials are now seeking federal construction aid and bids for a public-private partnership for a team of companies to build the $2.37-billion line and then operate and maintain it for about 25 years.