With a staff of about 220 assistant U.S. attorneys, the U.S. Attorney's Office for the Southern District of New York is one of the largest in the country. But that prosecutorial firepower isn't the only reason why it has become among the most sought-after assignments for aspiring federal prosecutors.

The office, which handles some of the highest-profile cases in the country, has developed such an independent streak that within the Justice Department, it's earned the nickname "the Sovereign District of New York."

That's why any mention of a witness cooperating with the office — as ex-Trump attorney Michael Cohen said publicly in testimony on Capitol Hill last month — should inspire fear in potential subjects of that assistance, former federal prosecutors say.

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Cohen's cooperation "would send a chill up my spine" if he worked at the White House, former New Jersey Republican Gov. Chris Christie, who also was a federal prosecutor, told ABC News during the hearing, referring to the ex-Trump fixer's statement to lawmakers that he was helping prosecutors in their Trump-related investigations.

What's more, prosecutors in Manhattan's Southern District aren't saddled with a limited mandate like special counsel Robert Mueller in his Russia probe; they can follow the trail wherever it leads.

The office has "historically been very independent, with very little oversight by DOJ, especially in the financial crimes space," said Lynn Neils, a former assistant U.S. attorney in the Southern District and in New Jersey. That's the sphere where the office "has made its name," she added.

Many of its alumni have too. Two past Supreme Court justices, Felix Frankfurter and John Marshall Harlan, former FBI Director Louis Freeh and New York Gov. Thomas Dewey, the 1948 Republican presidential nominee, and past Secretaries of State Henry Stimson and Elihu Root all worked at SDNY earlier in their careers.

A couple of those involved in the probes into Russian interference in the 2016 presidential election and whether there was coordination by President Donald Trump's campaign associates — Trump's attorney Rudy Giuliani and former FBI Director James Comey — also helmed the Manhattan office.

Giuliani — who as Trump's personal attorney has ridiculed Comey — hired him to join his team of prosecutors in the Southern District in 1987.

"He was like a choir boy who turned into a devil," Giuliani said last year. In his memoir "A Higher Loyalty," Comey both expressed admiration for Giuliani and criticized his leadership style, saying he made the office too much about himself.

That so many prominent people would come from one U.S. attorney's office is no surprise to those who've worked there.

"It is incredibly competitive to get a position there, and it attracts the best lawyers from all parts of the country," Neils said, adding that prosecutors in the office "are known for being really smart, hardworking and aggressive."

Those prosecutors earned their reputations on some of biggest criminal cases of the past century, from the espionage trial of Julius and Ethel Rosenberg, convicted of passing nuclear secrets to the Soviet Union, to the financial crimes of Bernie Madoff, who was sentenced to 150 years in prison in 2009 for swindling tens of billions of dollars from unsuspecting investors in the largest Ponzi scheme in history.

The Rosenbergs, who were executed in 1953, were convicted of delivering nuclear secrets to the Russians at a time when the United States was the only country to have the weaponry. The couple's trial in 1951 was the most prominent spy case in American history.

Thirty years later, the Manhattan U.S. attorney's office was routinely making front-page news under Giuliani's leadership. Giuliani actually took somewhat of a demotion from a high-ranking position in then-President Ronald Reagan's Justice Department to run the Manhattan office — a move he himself pushed for.

It was there that the future mayor of New York became a semi-household name for prosecuting the mob and going after Wall Street figures like Ivan Boesky and Michael Milken. As U.S. attorney, Giuliani amassed more than 4,000 convictions and had 25 reversals. He is even credited with popularizing the "perp walk."

In what was known as the Mafia Commission Trial, which ran for most of 1985 and 1986, Giuliani indicted the heads of New York's "Five Families" under the Racketeer Influenced and Corrupt Organizations Act. Giuliani's efforts led to the imprisonment of bosses of the Genovese, Colombo, Lucchese, Bonanno and Gambino crime families. Colombo and Gambino leaders Carmine "the Snake" Persico and John Gotti even encouraged the assassination of Giuliani, who has said Sicilian mafia heavyweight Salvatore Toto "the Beast" Riina sought an $800,000 hit on him.

Giuliani's most prominent Wall Street prosecutions were of Boesky and Milken. Boesky was prosecuted on insider trading charges, later providing information on others like Milken, who was charged under the RICO Act with racketeering and fraud.

Rudy Giuliani speaks to reporters at Trump Tower in New York on Jan. 12, 2017. Drew Angerer / Getty Images file

Comey, who served as the Manhattan U.S. attorney briefly in the early years of President George W. Bush's administration, is perhaps best remembered for prosecuting Martha Stewart for securities fraud and obstruction of justice as a result of a stock trading scandal. But also prominent was his prosecution of John Rigas, the founder of now-defunct cable giant Adelphia Communications Corp., for bank, wire and securities fraud.

More recently, under Preet Bharara, who was fired by Trump along with dozens of other U.S. attorneys early in his presidency, the office prosecuted New York State Assembly Speaker Sheldon Silver on public corruption charges. He was initially sentenced to 12 years in prison and ordered to pay out more than $7 million in forfeiture and fines. Though his conviction was overturned, he was later sentenced to seven years in prison during a retrial in which he was found guilty of the same charges.

The office's distinguished history gives it "a kind of street credit in the prosecution world that does give them some sense of independence from the Department of Justice," said Roland Riopelle, a former assistant U.S. attorney in the office.

In addition to its prosecution of Cohen for a litany of crimes — including a pair of campaign-finance violations for hush payments he says he made to two women at Trump's direction shortly before the 2016 election to keep them from talking about alleged affairs with the then-presidential candidate, affairs that Trump has denied — the office is probing the Trump Organization and Trump's inaugural committee.

FBI Director James Comey at FBI headquarters on June 23, 2014 in Washington, DC. Mark Wilson / Getty Images file

The New York Times reported in February that late last year, Trump asked then-acting Attorney General Matthew Whitaker whether Geoffrey Berman, the Trump-appointed U.S. attorney in Manhattan, could be put back in charge of the investigations involving Cohen. Berman recused himself from those probes. Although there was no indication Whitaker did anything as a result of Trump's request, Whitaker told colleagues that Berman's office needed "adult supervision," The Times reported.

Mimi Rocah, a former assistant U.S. attorney in the office and a legal analyst for MSNBC and NBC News, said that compared to other U.S. attorney's offices, Manhattan's Southern District bristles at substantial involvement from the Justice Department.

"Southern District has this deep bench of experience in so many areas, so we think 'we know how to do this better than anybody, so we don't need that type of oversight,'" she said, adding that the office will still follow standard Justice Department policies in any Trump investigations no matter its level of independence.

That includes the Office of Legal Counsel memo stating that a sitting president cannot be indicted.

"They are not going to just buck that on their own," Rocah said. "Will they push against it? Maybe. If the facts are warranted, and I don't know right now if they are, but you could imagine a world where they had facts that were."

Harvard Law School professor emeritus Alan Dershowitz, whose legal analysis defending Trump has been praised by the president, said the office's independence is "one of the reasons" the Manhattan investigations provide Trump with "much more to worry about" than the Mueller probe.

"New York has always had a degree of independence, and that's a reality," Dershowitz said. "And I think the U.S. attorneys can take advantage of that."

"They still have to run their indictments up through the hierarchy and get approval," he added. "No member of the Trump family is going to be prosecuted without it going through the Justice Department."