It's nightfall in Washington, D.C., at the end of the evening shift, when the throngs of students on school field trips have slowed to a trickle at the Vietnam Veterans Memorial.With a flashlight in one hand and a clear plastic bag in the other, Bob Herendeen walks the length of the austere, black granite wall. The National Park Service ranger surveys the things visitors have left at the memorial: American flags, wreaths, flowers.On this spring night, there's something else left behind. To the untrained eye it doesn't look like much — a few teaspoons of dirt or ash. But after six years working as a park ranger at the memorial, Herendeen immediately recognizes what it is: human remains.About once a week, Herendeen says, he comes across scattered human ashes at the wall, inscribed with the names of 58,318 U.S. service members believed to be dead or missing after the Vietnam War.It's a difficult part of his duties. More personal items, like medals and dog tags, are gathered up and curated; some become part of the memorial's permanent collection. Unlike those mementos, the cremains can't be added to the collection."You realize on the one hand that this was somebody's last wish to have their remains placed here on the wall," he says. "The law does not allow us to do that, so we have to follow a protocol, clean up the ash and remove it. It has to go into a hazardous waste disposal."The memorial was dedicated in 1982, and people have left cremains there — either scattered or in containers — since 1990.But for the last couple of years, it has been happening more frequently. The generation that served in Vietnam is aging — the youngest combat veterans are now in their 60s — and there was a particularly noticeable uptick after the 10-part documentary The Vietnam War began airing on PBS stations in September 2017.Now, the National Park Service, which is in charge of the memorial and the associated collection, is trying to discourage people from leaving cremains at the wall. Last fall, a new sign went up, warning visitors that the scattering of human remains is prohibited and that the ashes will not become part of the permanent collection. Visitors to the website of the memorial's permanent collection are given the same warning.The park service says it isn't equipped to care for human ashes and it is in discussions with veterans' organizations to find a permanent home for the cremains in its possession.While scattered ashes are disposed of, containers of ashes are taken to the vast warehouse at the National Park Service's Museum Resource Center in Landover, Md., outside Washington.There, items left behind at the wall are stored on rows upon rows of shelves: carved walking sticks, footballs, sculptures and blue boxes containing photographs, letters and other mementos. The collection holds as many as 300,000 items, collected from the foot of the memorial since its dedication, many of them tied to names on the wall.But the cremains are not part of that permanent collection."The things we have here are objects that tell a story," says Janet Folkerts, curator for the Vietnam Veterans Memorial museum collection. "But those [remains] are people, and we are not a mausoleum, and we are not a crematorium or a gravesite, so we don't have the capacity to take care of them in the way they should be taken care of."The ashes and their containers are kept separate from the rest of the collection, in a locked cabinet, its glass windows covered with brown paper for privacy. "So we don't have people just walking by and ogling," says Folkerts.Inside the cabinet are all sorts of containers: traditional urns, tins that look like they might have once held chewing tobacco, carved wooden boxes, a bandoleer with little glass vials of ashes in place of ammunition. There are at least partial remains of about 70 people. Some are marked with names, some are anonymous. There are poems and epitaphs — not all of them reverential. One of them reads: "My wife has a drinking problem: me."There's a letter attached to one of the containers, written in cursive on flowered stationery and addressed to James B. Elder, one of the soldiers whose names are on the wall, a casualty of the war. It reads: