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Robert Morgenthau, Manhattan's district attorney since 1974, announced Friday that he won't run for re-election this year, saying "enough is enough" after decades of locking up murderous mobsters, corrupt CEOs and thousands of other criminals.

Morgenthau, who will turn 90 in July, not only has been at the job since President Ford was in office, but he was the bustling borough's top federal prosecutor under Presidents Kennedy and Johnson.

"Some people are slow to learn. It took me a long time to realize I was getting older," he said.

His wife was by his side at a news conference, often repeating questions from reporters because he's hard of hearing. But he was upbeat, pondering life as a retiree.

"I don't know what I'll do," Morgenthau said. "I got an e-mail from my older brother who said this is a bad time to be looking for a job."

Morgenthau said the nation's busiest and most prominent district attorney's office has seen nearly 3.5 million cases in the 35 years he has led it.

Morgenthau cultivated a dignified, above-the-fray presence, and was widely acknowledged by allies and foes alike as effective, nonpartisan and incorruptible. Police Commissioner Raymond Kelly called him "an icon" and a "legend."

"He is without doubt the finest and most accomplished district attorney in our nation's history, a devoted family man and a hero to our great city," said Assembly Speaker Sheldon Silver, a Manhattan Democrat.

Tall and distinguished, Morgenthau was the model for the original prosecutor on "Law and Order," the long-running TV drama that features fictionalized versions of cases handled by his office.

In 2005, at age 86, Morgenthau was elected for the eighth time, turning back a challenge from Leslie Crocker Snyder, a popular former state judge who tried without success to turn Morgenthau's age and lengthy tenure into campaign issues.

Crocker Snyder is seen as a favorite to win the post in the November election.

"I wish Mr. Morgenthau well. He has been a great institution for New York, and I hope he is happy in the next phase of his life," she said. "I've been actively running, and I will be running."

Morgenthau prosecuted notables from John Gotti to Sean Combs to Boy George, and he hired young lawyers such as John F. Kennedy Jr., Eliot Spitzer and Andrew Cuomo to work as assistant prosecutors.

Morgenthau was born in 1919 into a wealthy, prominent New York family. His grandfather, Henry Morgenthau Sr., was U.S. ambassador to Turkey during World War I, and his father, Henry Morgenthau Jr., was secretary of the treasury under President Franklin D. Roosevelt, a family friend.

His childhood reflected his lineage. Morgenthau had a lifelong friendship with members of the Kennedy clan; he once cooked hot dogs with Eleanor Roosevelt for Great Britain's King George VI; on another occasion he prepared a mint julep for Winston Churchill.

In 1960, Morgenthau campaigned in New York for his friend and fellow Democrat, John F. Kennedy. The next year, the new president named him to the prestigious post of U.S. attorney for the Southern District of New York, the nation's busiest such office.

He served in that office through the Kennedy and Johnson administrations and ran for district attorney in 1974.

Morgenthau's notable convictions as district attorney included a crack dealer who murdered the son of AOL Time-Warner head Gerald Levin; seven youths who killed a Utah tourist in a subway mugging in 1990; and L. Dennis Kozlowski, the former Tyco CEO sent to prison after a lengthy corporate theft trial.

He also prosecuted Bernhard Goetz in the 1984 wounding of four black youths who tried to rob him on a subway train, in a case that became symbolic of vigilante justice during the crime-ridden 1980s. He also jailed Robert Chambers Jr. in what became known as the "Preppie Killer case."

His office boasted a strong conviction rate, although it wasn't perfect. A jury cleared Gotti during one of his many brushes with the law in the 1980s, and Combs was acquitted in a gun case earlier this decade.

One of the most sensational prosecutions handled by his office was the Central Park jogger case. Thirteen years after a female jogger was attacked in the infamous "wilding," Morgenthau asked a judge in 2002 to throw out the convictions of five men because DNA evidence and another man's confession put them into question.

Not everyone was full of praise for Morgenthau.

"I haven't felt so bad since Nixon resigned," defense lawyer Ronald Kuby said, chuckling. "Morgenthau held the office longer than he should have. ... Of the five district attorneys (in New York City), his office was the most difficult to deal with."