When conservationists trying to reintroduce black vultures to Bulgaria saw Ultron leave his artificial nest on Kotlenska Mountain for the first time on August 15, they didn't know if he'd leapt or fallen.
So they breathed a sigh of relief when, after some tumbles as he got the hang of his wings, the 3-month-old raptor returned to feed at Kotlenska five days later, seemingly flying confidently.
A few months later, the groups behind the international Bright Future For The Black Vulture project marked another milestone when Belgian-born Ultron joined more of the rare birds -- extinct as a "nesting species" in Bulgaria since 1992 -- to wander the Balkans.
Closely following his movements via a GPS tracker, they watched as their young charge wintered in with the Balkans' only natural colony of black vultures, also known as cinereous vultures for their dark, cinderlike color.
But the environmentalists' hopes came crashing down in March.
That's when a hunter opened fire on Ultron in the Kardzhali region, about 75 kilometers from the Bulgarian-Greek border, during a layover for bad weather on his way "home" to Kotlenska, in northeastern Bulgaria.
"The satellite transmitter on the bird immediately sent data about the bird’s distress and our team went to check on the site," one of the groups behind the reintroduction project, NGO Green Balkans, said. "We found Ultron shot and covered in blood."
They took the bird to a nearby animal-rescue center, where a team of veterinarians spent days trying unsuccessfully to save Ultron.
Now those same conservationists warn that the tragedy could be a major setback for their cause in Bulgaria, as it could also discourage donations of rare animals like Ultron from German, Dutch, Belgian, and other foreign partners in the vulture and similar wildlife-return projects in the country.
"This is a huge blow to nature protection in Bulgaria, but also to nature in Bulgaria and the Balkans in general," Green Balkans said.
For decades, environmentalists have warned of the dangers to Bulgaria's wildlife habitat from an explosion of tourism venues, golf courses, and ski resorts, in addition to sand and gravel extraction and runaway lumber production.
Kotlenska Mountain and its deciduous forests and pastures were labeled an "Important Bird Area" by BirdLife International in 1997.
The area is home to more than 180 species of birds, 51 of them in the International Union for Conservation of Nature (IUCN)'s Red List of Threatened Species for Bulgaria.
Black vultures are listed as "near threatened" on the IUCN list, with fewer than 5,000 mature birds globally.
Fueled by a desire to see Ultron's killer punished and demonstrate that Bulgaria can protect its at-risk animals, conservationists went public about Ultron's death on May 25 to pressure prosecutors to investigate it and seek tips from the public to find the killer.
"The reason for announcing the information is that nothing has been happening for more than two months," Ivelin Ivanov, the New LIFE For Vultures project manager at Green Balkans, told RFE/RL's Bulgarian Service.
"The bird was downed in March. We immediately signaled it [and] we were told it was in the works. Meanwhile, Ultron underwent surgery and died. We called again -- nothing. Two months have passed already, and the institutions are still silent," he said.
The day after his group's press release, Kardzhali district prosecutors issued a press release of their own, announcing a criminal investigation and the collection of evidence, citing an article of the penal code on killing or trafficking in animals listed under Bulgaria's Biological Diversity Act of 2002.
Infractions can be punished by up to five years in prison and a 5,000 to 20,000 leva fine ($2,863-$11,511), plus compensation.
Contacted by RFE/RL, the prosecutor's office declined to explain the delay in launching the investigation.
Ultron was shot on a 16,000-hectare hunting reserve, called Zhenda, in the low mountains of southern Bulgaria that mostly caters to hunters of wild boar, deer, and mouflon, the wild sheep whose males are prized for their spiral, fluted horns.
Green Balkans has been keeping Ultron's carcass preserved in anticipation of forensic examination.
But "two months after what happened, I don't know what they will find during the investigation," Ivanov said. "All traces have long since been erased from the place where Ultron was shot."
The conservationists have suggested that authorities seek data from mobile-phone operators to find out who was in the area at the time of the shooting, something that couldn't have been done before starting the criminal investigation. They are also trying to help determine if closed-circuit cameras in the area might provide clues.
The Bulgarian black vulture effort that brought Ultron to Kotlenska is one of several trying to reconnect now-distinct populations across Europe and Eurasia.
The country is already home to the griffon vulture, whose successful reintroduction 70 years after it had disappeared helped inspire the black vulture project.
Ultron was a gift from the Planckendael Zoo in Belgium under the European Endangered Species Program and the Bright Future For The Black Vulture project.
He was one of two male black vultures born abroad and released into the Bulgarian wild in August as 3-month-old nestlings -- and one of fewer than a dozen individuals freed there in the past 15 months.
The local Fund For Wild Flora And Fauna (FWFF) group listed three goals for Bulgaria when it began its black vulture project in 2014: getting more than one vulture in the area at any one time; ensuring a year-round presence of at least one animal, rather than just stopovers; and ensuring a breeding attempt, which would mark a shift from locally "extinct" to "critically endangered."
The FWFF said between five and 15 black vultures were seen in the area since releases from eastern Balkan Mountain aviaries started in March 2019, and a year-round presence was cited earlier this year.
That left only local breeding still to be established.
But instead of a romantic ending for Ultron and his fellow black vultures, a poacher's gun has confronted Bulgarian conservationists and investigators with a whodunit whose trail may have already gone cold.