NEW YORK, June 7 (Reuters) - The top federal prosecutor in New York said during a speech Monday evening that it was fair to question why so few prosecutions have resulted from the financial crisis, but he added that bringing such cases takes time.

"It is a function of the law, the facts, and the painstaking nature of these investigations," said Preet Bharara, the U.S. Attorney for the Southern District of New York, during an event Monday sponsored by the New York Financial Writers Association at the the CUNY Graduate School of Journalism.

Bharara remarked that even his own family had asked him about the number of cases that have been brought in the wake of the nation's worst financial crisis since the Great Depression.

He said the figure is not a function of prosecutorial resources or commitment, noting that it took more than four years to investigate and bring Galleon Group founder Raj Rajaratnam to trial on charges of insider trading. A jury in Manhattan federal court convicted Rajaratnam on all fourteen counts of the indictment last month.

During the speech, which covered the mission and recent achievements of his office, Bharara also said that large, successful companies should not automatically get immunity from prosecution.

"We shouldn't be telling any institution that it is too big to prosecute," he said.

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Since taking office in August 2009, Bharara's office has made some of its biggest splashes by prosecuting insider trading cases. During his tenure, 49 individuals have been charged with insider-trading crimes; 39 of those have pleaded guilty or been convicted by a jury.

A federal jury in Manhattan is currently deliberating in a case against a former Galleon trader and two other traders who are accused of conspiring to trade on tips about mergers from the law firm Ropes & Gray.

In its insider-trading investigations, Bharara's office has made extensive use of wiretaps, which have traditionally been used for cases involving drugs and organized crime. When Bharara announced the charges against Rajaratnam in October 2009, he said it was the first time wiretaps had been used to target major insider trading on Wall Street.

Bharara said he thought the cases were having a deterrent effect. Someone who engages in insider trading today is "especially foolish or pathological," he said.

(Reporting by Andrew Longstreth)