The day Mr. Buckel died, Mr. Morales received a text from him at 5:25 a.m. saying he wouldn’t be at work. At 5:55 a.m., he got the letter Mr. Buckel emailed to several news organizations. He was arriving at the Red Hook site and dismissed it at first, taking it for one of the news stories they’d exchange.

“I sent him a text at 6:40 saying, ‘Please tell me that email is just a joke.’”

Above Mr. Buckel’s letter, there was a note. “I apologize for leaving this world early and leaving you with some big challenges to tackle,” it said. “You are ready. I am so proud of you and what you have accomplished, both in your professional and your personal life. It was an honor and a pleasure to serve the Earth with you.”

Mr. Morales, not wanting to believe what he had read, showed it to a few other workers later that morning. He did a Google search for “self-immolation.”

“It popped up — Prospect Park,” he said. “We just dropped to the ground.”

Mr. Buckel chose to die not before thousands, but alone. He chose to do it at an early hour, perhaps so the sight of a burning man would be less likely to traumatize children, or be caught on video.

Even the location, that forgettable strip of grass just off the road, seemed intended to disrupt life as little as possible. “I apologize to you for the mess,” he wrote in a note to the police, which was found with his letter in the shopping cart. He had stapled his business card to the letter, and left his identification on a lanyard nearby, to remove any doubt.

He kept his cellphone with him, which was found in melted pieces by his body, along with a knife and some keys.

Among the many unanswered questions about the death of David Buckel was why he had a folding shopping cart with him. There was no fuel canister in it, nothing but an empty black plastic bag — the kind he and Mr. Morales filled with the rich new soil made on their compost site.

The earth around Mr. Buckel was burned in a nearly perfect circle. The police said the ground was too scorched to tell, but it is possible that when he went to Prospect Park that day, he took some soil with him, hauling it in the cart. It is possible Mr. Buckel’s last moments were spent spreading it out, making a ring around himself, so the flames wouldn’t spread.