The bucolic Mason Neck peninsula on the Potomac is an area where usually the only sounds to break the silence are bird calls and the lapping of the river, but shortly after 1 a.m. Friday, gunfire erupted at one of the upscale homes in that area of Fairfax County, police said.

A woman went downstairs to investigate and found her 83-year-old husband with several gunshot wounds, police said. Johan De Leede was rushed to a hospital and later died of his injuries.

The death of the retired World Bank economist and civic activist, best known for working for the construction of a local bike and pedestrian trail, stunned his neighborhood and spawned a mystery: Authorities said Friday that they had no suspects in the killing. Police dogs and a helicopter swept the isolated area but found no potential assailant.

“We have no credible evidence or leads at this point. It’s a true mystery,” said Fairfax County Police Chief Edwin C. Roessler Jr. “This truly is a case where we need input from the community. We want to hear about any suspicious activity.”

Neighbors, friends and former colleagues said they were equally baffled. They knew of no problems and doubted De Leede had any enemies. Relatives of De Leede’s declined to comment.

“His smiling face has always been a reminder of why me and my family moved to this part of the county,” said Robert Lee, the treasurer of the Mason Neck Citizens Association.

Police said De Leede’s wife called authorities at 1:06 a.m., saying that her husband was unresponsive. A neighbor then called 911 to report gunshots in the area, which is in the 6000 block of River Drive. De Leede’s wife initially thought he had fallen, police said. They asked residents to remain indoors while they searched the area.

Tim Rizer, who lives near the De Leede home, said he awoke about 1:30 a.m. to the sound of police helicopters above the neighborhood of million-dollar homes that rarely sees any crime. The community — near where former Republican presidential candidate Carly Fiorina lives — jealously guards its serenity. Most of the peninsula is parkland.

“Generally, in our neighborhood, if there are helicopters, you figure there was some accident on the Potomac River,” Rizer said. “I got up, looked around, didn’t see anything, and the helicopters kept flying around over and over and over again, and I thought, ‘Well, maybe somebody is on the loose.’ ”

Police said detectives have been interviewing De Leede family members and neighbors. Roessler said the investigation at the scene could last days as investigators scour the property for clues. River Drive remained blocked Friday, and police could be seen working on the rear deck of the De Leede home.

Roessler said he could not declare that there was no further danger to the public.

“I don’t have any credible information there’s a danger to the community,” Roessler said. “We don’t know if this was targeted or random. We need the community to be aware of their surroundings until we verify whether there was a relationship between the killer and the victim or not.”

A spokesman at the World Bank said De Leede worked there for nearly 25 years before retiring as a senior economist in 1994. Mustafa Soykan, a friend and former colleague, said De Leede’s work focused on French-speaking countries in Africa. Soykan said that after De Leede retired, the two played golf together and that his friend was enjoying his life, spending time with his four children and their children.

Joe Chudzik, a neighbor and friend, said De Leede was an outdoorsman who biked, skied and kayaked. The passion was also evident in his civic work. Chudzik said De Leede had conceived and begun pushing for the Mason Neck Trail in 1995. For part of two decades, De Leede advocated for construction of the six-mile, roughly $5 million bike and pedestrian route that connected denser areas of Lorton with the parks on the Mason Neck peninsula. The trail is still under construction.

Chudzik said that when De Leede wasn’t working on that project, he participated in adopt-a-road cleanups.

“He gave back to the community,” Chudzik said.

As he grew older, De Leede became less active in those efforts but continued to take his boat onto the river and regularly taught kayaking and light sailing to friends and neighborhood children.

“He built a nice little area out in the back of his house on the water that had sand and stuff,” Rizer said. “You’d see five or six people, usually kids or something, and he’d be showing them how to sail in the back of our houses.”

Some neighbors said they liked De Leede because of his strong advocacy for bike trails and because he took pride in caring for his house on the river’s edge.

“He’d be out there doing a little bit of gardening all the time,” said neighbor Mike Ostergard. “He was a healthy 83-year-old.”

Richard Kennedy, another neighbor, said De Leede and his wife were originally from the Netherlands. Kennedy said he, like everyone else, is stunned at the killing and wonders what might have happened.

“We’ve sent an email out to the neighborhood and asked whether anyone knew of anything out of the ordinary, but no one has replied,” Kennedy said. “It’s hard to imagine a person like him would have any enemies. It is pretty incomprehensible.”

Jennifer Jenkins and Dana Hedgpeth contributed to this report.