It is a violation of longstanding federal law to collect parts from the carcass of a protected marine animal if there are still “soft tissues” attached.

It becomes political intrigue if the collector was Robert F. Kennedy Jr., and the severed head of a possibly protected marine mammal streamed “whale juice” down the side of the family minivan three decades ago.

On Monday, the political arm of the Center for Biological Diversity, a progressive environmental organization, called on federal authorities to investigate an episode, recounted by Mr. Kennedy’s daughter in a 2012 magazine article, in which she said Mr. Kennedy chain-sawed the head off a dead whale on a beach in Hyannis Port, Mass., bungee-corded it to their vehicle’s roof, and drove it five hours to the family home in Mount Kisco, N.Y.

“It was the rankest thing on the planet,” Kick Kennedy, then 24, told Town & Country, in the article, which described Mr. Kennedy as someone who likes to study animal skulls and skeletons.

The story recently resurfaced, including in an entertainment publication, The Wrap, on Sunday and in a New York Post article on Monday.

In a letter to the National Oceanic and Atmospheric Administration, which oversees marine protection, Brett Hartl, the national political director for the Center for Biological Diversity Action Fund, wrote: “There are good reasons why it is illegal for any person to collect or keep parts of any endangered species.”

“Most importantly, vital research opportunities are lost when individuals scavenge a wildlife carcass and interfere with the work of scientists. This is particularly true of marine mammals, which are some of the most difficult wildlife species in the world to study.”

Some species of whales, Mr. Hartl noted, are so rare that scientists can learn about them only through their carcasses washed ashore.

Mr. Kennedy, he wrote, may have violated not only the Marine Mammal Protection Act (1972) and the Endangered Species Act (1973) — two seminal marine conservation laws — but also the Lacey Act of 1900, a conservation law signed by President William McKinley that prohibits the transportation of illegally gathered wildlife, dead or alive, across state lines.

Mr. Kennedy did not respond to a request for comment. Representatives for NOAA did not respond to a request for comment.

The whale has now joined a baby bear, at least one emu and a worm whose deaths have been intimately associated with Mr. Kennedy, the independent presidential candidate — and environmental lawyer — who last week joined forces with former President Donald J. Trump’s campaign.

The Center for Biological Diversity Action Fund has endorsed Vice President Kamala Harris for president.

Mr. Kennedy, 70, is a lifelong collector, trainer and handler of wild animals. At his home in Los Angeles, where he has been taming a pair of wild ravens, he also keeps a taxidermied turtle (a former pet) and a rare Sumatran tiger (a gift to his father). Other former pets include at least two emus, one of whom was killed by a mountain lion, the other by a family dog.

This month, Mr. Kennedy acknowledged that he had left a dead bear cub in Central Park in 2014 because he thought it would be “amusing” — an admission that came ahead of a piece in The New Yorker that detailed the episode. Mr. Kennedy told Roseanne Barr that he found the dead bear and had planned to skin the bear and put the meat in his refrigerator, but instead staged it with a bicycle in Central Park.

While some of his pursuits with wildlife have been merely curious, some may have skirted the line of the law. The North American Migratory Bird Treaty Act, for example, prohibits the “capture” of at least two species of ravens. New York law prohibits the “illegal possession” and the “illegal disposal” of a bear.

In an interview on Tuesday, Mr. Hartl said the whale incident, if true, would represent a “far worse” violation than the bear cub. (And if Mr. Kennedy still has any part of the whale’s skeleton, he could still potentially be charged with a federal offense even 30 years later, Mr. Hartl said.)

“When you swear into the bar” as a lawyer, “you take an oath to uphold the law,” said Mr. Hartl, noting that he, like Mr. Kennedy, is an environmental lawyer. “This isn’t a stupid rule. This is the law — laws that were passed almost unanimously” decades ago, he said. “This is not some silly current thing of the ‘corrupt deep state’ that he now claims to care about.”