He had set himself on fire, the note said, in a protest against environmental destruction.
Buckel, who was 60, left a note in a shopping cart near his body and emailed copies of the letter to several news outlets, according to the New York Times.
He had spent the bulk of his career fighting for gay rights, including serving as the marriage project director and senior counsel at Lambda Legal, a nonprofit civil rights organization. He was the lead attorney in the lawsuit involving Brandon Teena, a transgender man who was gang-raped and murdered in 1993 in Nebraska, according to the Associated Press.
"David was an indefatigable attorney and advocate, and also a dedicated and loving friend to so many. He will be remembered for his kindness, devotion, and vision for justice, Camilla Taylor, Lambda Legal's director of constitutional litigation and acting legal director said in a statement. "Our hearts go out to all who knew David. He was a beautiful human being who was universally kind to everyone at Lambda Legal, committed to his clients, and devoted to our work."
Buckel had been the strategist behind same-sex marriage cases in New Jersey and Iowa, according to the organization, prompting the Hawkeye state to become the third to legalize same-sex marriage in 2009.
In another case he led, the New Jersey Supreme Court ruled that same-sex couples and their children were harmed because they were prevented from having rights granted from marriage.
"We have lost a movement leader, a colleague, and a friend," Taylor's statement said. "We will honor his life by continuing his fight for a better world."
After Buckel left Lambda Legal, he had become heavily involved in environmental causes, which he alluded to in his final note.
"Pollution ravages our planet, oozing inhabitability via air, soil, water and weather," he wrote in the email sent to The Times. "Most humans on the planet now breathe air made unhealthy by fossil fuels, and many die early deaths as a result - my early death by fossil fuel reflects what we are doing to ourselves."
His final note said he had "good health to the final moment," but wanted others to be spurred to action, according to the Times: "Honorable purpose in life invites honorable purpose in death."
Conversely, the note said, privilege was derived from the suffering of others and donating money was not enough to cure society's ills.
"Many who drive their own lives to help others often realize that they do not change what causes the need for their help," he wrote.
He likened his self-immolation to protests by Tibetan monks making a powerful political statement against Chinese occupation, according to HuffPost.
"This is not new, as many have chose to give a life based on the view that no other action can most meaningfully address the harm they see," Buckel wrote.
"Here is a hope that giving a life might bring some attention to the need for expanded actions, and help others give a voice to our home, and Earth is heard."
(This story has not been edited by NDTV staff and is auto-generated from a syndicated feed.)