John Moscow, the prosecutor who oversaw the fraud trial of the former chief executive of Tyco International Ltd., L. Dennis Kozlowski, is leaving the New York District Attorney’s Office to enter private practice.

Mr. Moscow, who heads District Attorney Robert Morgenthau’s investigations division, won 16 convictions in the Bank of Credit and Commerce International case in the 1990s and helped convict the chief financial officer of brokerage A.R. Baron & Co. in 1998.

His office is known for prosecuting international defendants with New York state laws, including a securities fraud law called the Martin Act that New York Attorney General Eliot Spitzer has used against Wall Street firms, mutual funds, and insurance companies.

“State laws exist, which people tend to forget,” Mr. Moscow said in an interview. “If you don’t engage in wrongful conduct, people aren’t going to be looking for technicalities, but if you’re looting a place, they’re going to be looking for ways to get you.”

Mr. Moscow is leaving three weeks before the re-trial of Mr. Kozlowski and Tyco’s former chief financial officer, Mark Swartz, is set to begin. Their first, six-month fraud trial ended in April when the jury couldn’t reach a unanimous verdict. Another indicted Tyco executive, the former chief counsel Mark Belnick, was acquitted.

“The Tyco trial had absolutely nothing to do with John deciding to leave,” said Tom Curran, who worked with Mr. Moscow for three years and counts him as a friend. He called Moscow a “legal Boy Scout” who “used the grand jury mechanism for investigative purposes like a virtuoso.”

Mr. Morgenthau’s spokeswoman, Barbara Thompson, declined to comment on Mr. Moscow’s departure.

In 1972, Mr. Moscow joined the district attorney’s office, then headed by Frank Hogan, right after graduating from Harvard Law School. His father, the reporter Warren Moscow, had worked for Hogan’s failed 1958 Senate campaign.