There’s a maxim among homicide investigators that if a killing isn’t solved within the first 24 hours, the chances of solving the case drop precipitously.

So how about 25 years?

Sunday marks a quarter-century since Assistant U.S. Attorney Thomas Wales was gunned down while sitting at a computer in the basement of his Seattle home, the shots fired through a window by a killer who avoided backyard security lighting. The assassination set in motion one of the most intense — and frustrating — investigations in the country’s history.

At a news conference Friday commemorating the anniversary, First Assistant U.S. Attorney Charles Neil Floyd and FBI Special Agent in Charge Mike Herrington insisted that the investigation is active despite the passage of time.

As in past news conferences — roughly one every five years — they emphasized that there are members of the Western Washington community who have information about the killing, likely including the gunman.

For anyone who has been withholding information that might break the case, Floyd enticed them with a $2.5 million reward: “This is life-changing money, and we are serious about it,” he said.

“After 25 years, people may incorrectly believe that this case is cold,” Herrington said. He said technologies that didn’t exist in 2001 have been brought to bear, and noted that the FBI is known for tenacity.

“This is what we do,” he said.

Floyd also made a plea to the shooter, who is believed to still be in Western Washington, possibly in Snohomish County.

“You have carried a heavy, secret burden,” he said. “We urge you to do the right thing.”

Amy Wales, Tom Wales’ daughter, who in the past has spoken for the family, was absent from the news conference but issued a written statement.

“Although I have now lived more years without him than with him, I remain profoundly grateful for the time we had together,” she wrote. “Justice for my father is overdue, but the commitment of the people gathered at today’s press conference strengthens my resolve and keeps me hopeful.”

After a slow start, the FBI eventually dedicated a task force code-named “SEPROM” (for “Seattle Prosecutor Murder”) manned by more than a dozen agents with a budget of its own. The bureau’s response to Wales’ death was initially muted because almost all of its resources and attention were focused on the terrorist attacks of Sept. 11, 2001, just a month earlier. By the time the Wales case geared up, chances of solving it had diminished.

The task force has since dwindled to two, according to Assistant U.S. Attorney Thomas Woods, who oversees its operation.

There was confusion and mistakes at the crime scene, where evidence was overlooked and jurisdictional squabbles between federal agents and Seattle police resulted in critical delays.

Critical witness interviews were neglected. It was almost three years after the shooting when the task force learned that a neighbor had repeatedly seen a suspicious man in the area just before the shooting. Agents released a composite sketch of that person — now thought to be connected to the killing — long after the fact.

They did collect some evidence from the scene that led to a dramatic international search for a specific handgun — a Russian Makarov with a replacement barrel. At one point, agents raided a Bellevue gun collector they thought may have provided the shooter with the weapon used to kill Wales.

The FBI took the lead in the case because the 49-year-old Wales was a federal prosecutor and evidence early on pointed to the likelihood that his death was related to his work. If that is the case, then Wales remains the first federal prosecutor in U.S. history to be killed for doing his job.

Indeed, police, agents and Wales’ colleagues focused their attention within hours on a particularly contentious prosecution undertaken by Wales, who specialized in white-collar crime.

The case involved a company and its principals who were indicted on a charge of allegedly refitting military helicopters into civilian models, in violation of federal aviation law. Wales pursued the company and its owners relentlessly for two years before the criminal charges were dismissed and the company fined.

One of the company’s principals, an airline pilot living in the Bellevue area, was bitter and threatening and filed a wrongful prosecution lawsuit against Wales, which was ultimately dismissed. Pleadings in that case identified the pilot as a suspect in Wales’ death.

The Seattle Times has never identified the pilot because he has never been charged.

However, he remains the key suspect to this day, although the FBI’s theory of the case has changed significantly since the early days of the investigation. Initially, investigators suspected the pilot had traveled to Wales’ home in the Queen Anne neighborhood, shot Wales and then sped back to his home in Beaux Arts Village, where he was known to be on the telephone later that evening.

Agents have searched his homes repeatedly, had him pulled from the cockpit, dragged friends and family before a grand jury and run an undercover sting operation.

They’ve spent years tracking the buyers of custom gun barrels connected to the crime scene and pressured and even indicted people they believe were bit players in the plot in hopes of breaking the case, but to no avail.

In recent years, the focus has turned away from the pilot as the killer and instead leaned into the likelihood that the pilot arranged for a hit man, possibly connected to drug cartels, to carry out the assassination.

Agents believe the pilot may have been smuggling drugs for the cartel and sought its help in finding someone to kill Wales. The working theory now is that a low-level drug dealer out of Snohomish County, in debt to the cartel, was coerced into shooting the prosecutor.

In all the years, they made one arrest: a Snohomish County woman the Department of Justice accused of lying to a grand jury. She eventually pleaded guilty to a misdemeanor obstruction charge and provided no useful information to the task force.

Correction: A previous version of this story misstated First Assistant U.S. Attorney Charles Neil Floyd’s title. It has been updated.