Kevin B. Lake knew he likely was spending the last days in his colonial-style, red-brick house on the verdant edge of New Albany.

A five-year sentence in a federal prison was looming.

Lake, 51, was getting his affairs in order after pleading guilty to drug, fraud and tax-evasion charges. His license to practice osteopathic medicine had been revoked.

Federal prosecutors had seized nearly $30 million from Lake's personal accounts and his business, the former Columbus Southern Medical Center at 2912 S. High St. Authorities said the clinic for years illegally prescribed painkillers in vast quantities.

But authorities said Lake had bigger, more serious concerns. After he had agreed to testify in future investigations as a condition of his plea agreement with the federal government, Lake started to receive threats, said Maj. Steve Tucker, a Franklin County Sheriff's Office detective.

At 7 a.m. June 22, Lake and his son, Jonah, were on the second floor of the large house on Schleppi Road when they heard the sound of a window being broken downstairs.

Shortly after Lake descended the stairs to investigate, his son heard gunshots from a first-floor bedroom, where he found his father mortally wounded.

Lake died the next day, leaving an unsolved puzzle for investigators.

"This one presents a number of challenges," Tucker said of the case. "I firmly believe we are on the right path. It's just going to take time."

The sheriff's office has called upon the FBI, Ohio Organized Crime agents, the Columbus Division of Police and the U.S. attorney's office for help.

Computers were seized from the house and investigators are combing through Lake's communications for clues.

Tucker wouldn't comment about what the evidence shows about how the attack was carried out or how many times Lake was shot.

For years, Lake had overseen a criminal enterprise that illegally prescribed addictive, painkilling drugs from the south Columbus clinic, Tucker said.

"There are a number of people who could have an ax to grind with something like this," he said.

Assistant U.S. Attorney Kenneth Affeldt, who helped prosecute Lake, said he cannot comment about whether Lake made any statements to federal investigators before his death. It is standard in most plea agreements for defendants to cooperate with authorities.

Bradley Davis Barbin, an attorney who represents the Lake family, said that around the time of Lake's death, the family was aware of threats related to his agreement to cooperate with the government. Barbin would not comment further.

One threat that detectives learned about was a letter with a photo someone took of Jonah Lake sleeping inside the home.

Lake's wife, Susan Lake, told detectives that she thought the person who took the photo and wrote the letter was trying to intimidate her husband from testifying, according to a search-warrant affidavit. The Lake family did not report that threat to authorities until after Lake's death.

Susan Lake, who also is a doctor of osteopathic medicine but didn't practice with her husband, had left for work the morning of the shooting.

Tucker would not reveal the contents of the letter. He said investigators believe the threat was authentic but added that he would not characterize it as a "death threat" because "there is a degree of ambiguity about the threat."

At some point, Tucker said, he would like to interview Susan and Jonah Lake again. Both gave statements to investigators on the day of the shooting, but their attorney has since denied investigators access to them.

For years, federal and state investigators dug below the surface of Lake's prosperous life as they unraveled the scams revolving around the Columbus Southern Medical Center.

"It was an interesting case with a lot of twists and turns," Affeldt said.

Lake graduated from Capital University in 1988 and received his osteopathic medical degree from Ohio University's College of Osteopathic Medicine in 1992. He began his medical career in 1994, and his association with the Columbus Southern Medical Center began in 1999.

Then-Gov. Ted Strickland appointed Lake to his medical school's advisory board in 2008. Strickland appointed Lake to the Ohio University board of trustees in June 2010.

It also was in June 2010 that federal Drug Enforcement Agency agents executed a search warrant as they started to examine the source of the osteopath's wealth: the Columbus Southern Medical Center.

When Lake first joined Columbus Southern in 1999, it was a family medical clinic with doctors who served south Columbus.

But the medical center's operation began to evolve in 2004 as records show a spike in painkillers prescribed to a growing clientele. In 2006, Columbus Southern converted into a pain-management clinic and Lake stopped treating patients directly in order to manage the enterprise, court records show.

"Often a line of patients would form before the clinic opened at 8 a.m., sometimes as early as 6:30 a.m. The waiting room was always packed, with many patients forced to stand," according to records in U.S. District Court in Columbus. Many of the patients would drive a considerable distance and pay the $100 appointment fee in cash.

Doctors and physicians assistants would each see up to 100 patients a day. Two doctors who worked with Lake -- Dr. Terry Dragash and Dr. David Rath -- along with Karen Climer, an administrative employee, pleaded guilty to federal charges of conspiracy to distribute oxycodone. Dragash and Climer served time; Rath died in January 2016 before sentencing.

When investigators started to put pressure on the clinic, Lake stayed in the background, eventually monitoring his employees from a remote location with his laptop. He would express his concern to the medical staff when they were "not seeing patients as quickly as he desired," court documents show.

After DEA agents started to examine Columbus Southern, authorities said Lake took steps to conceal his ownership and involvement in running the enterprise. Lake instructed his employees never to say he owned the clinic and kept his name off paperwork, according to court documents.

However, Lake had a number of corporations created, and he received proceeds from the clinic surreptitiously, court records show. From 2004 until the doors finally closed in May 2013, the clinic raked in $50 million in proceeds.

In addition, Columbus Southern improperly collected fees from the Ohio Bureau of Workers' Compensation for doctor visits. Lake agreed to repay $260,402 as part of his plea agreement.

On a personal level, Lake fraudulently claimed that he was disabled in 2010 to collect payments from private insurance. In 2012 and 2013, he collected state unemployment benefits.

The Internal Revenue Service also found that Lake fraudulently paid himself $90,000 for rental of his house for corporate retreats for his staff, which never occurred.

Out of the $29 million seized by the federal government, $9.5 million was sent to the IRS to cover unpaid taxes and penalties. Lake also forfeited four properties encompassing 464 acres of farmland and a property in Canal Winchester.

Affeldt said that the financial judgments would stand and final arrangements are being made.

During his plea-agreement hearing, it was acknowledged that Lake accepted responsibility for his actions with the goal of having a "clean slate" when his prison term was over.

But Lake never got that chance. And those investigating his slaying want to know why.