Leslie was not what anyone expected a bank robber to look like. He moved in high society, knew bank presidents personally, and used those connections to study the institutions he planned to rob. His method was meticulous to the point of obsession: he would become a depositor at a target bank, sketch architectural plans of its interior and exterior, then have his fence -- Fredericka 'Mother' Mandelbaum, one of New York's most powerful criminals -- provide a warehouse in Brooklyn where he built exact replicas of the banks for rehearsal. His signature tool was a device he called the 'little joker,' a mechanism that could reveal a safe's combination numbers by manipulating the tumbler mechanism. Police would later connect Leslie to the theft of $800,000 from Ocean National Bank in 1869 and $1.6 million from Northampton Bank in Massachusetts in 1876.

In early March 1878, Leslie broke into the Manhattan Savings Institution for the first time, using keys provided by Patrick Shevlin, the night watchman he had planted. He drilled through the safe's dial indicator and inserted a wire to throw off the combination gears, then worked through the night trying to crack it. He failed. Worse, his tampering jammed the safe so badly the bank had to install new tumblers, forcing him to start from scratch. On his second attempt, a passing police officer peered into the darkened lobby and spotted Leslie's associate Red Leary pretending to clean with a feather duster. Leary gave a thumbs up, and the officer left -- but the close call spooked them both. On March 15, the third break-in, Leslie pried off the dial knob and inserted an improved version of the little joker. This one did not need someone to enter the combination first. Spinning the knob revealed the tumbler positions directly: 80, 9, and 25.

The gang was already fracturing before the safe was cracked. In February 1878, Leslie and several associates had attempted to rob the Dexter National Bank in Maine. When the bank's cashier, James Barron, who had agreed to help them, tried to back out, two gang members cuffed and gagged him, wedging him between vault doors. Leslie tried to stop them but could not. Barron was found dead the next day. The Pinkertons opened an investigation. Back in New York, paranoia took hold. Thomas 'Shang' Draper and Johnny Irving feared Leslie would talk to save himself. Draper had his own grievance -- he had discovered Leslie's affair with his wife, Babe. Leslie gave Babe a pistol for protection after learning Draper had beaten her. Mother Mandelbaum tried to broker peace, but Draper and Irving were beyond persuasion. Leslie told his wife Molly he planned to take his share of the Manhattan robbery and move out west. It was the last time she saw him.

The gang struck at dawn. At 6:10 a.m., multiple men broke into the apartment of Louis Werckle, the bank's janitor, who lived nearby. They handcuffed Werckle and his wife, tied his mother-in-law to a bed with a sheet, and held a gun to Werckle's head until he gave up the safe combination. Using keys from the bribed night watchman Shevlin, four men entered the bank while the others guarded the family. Inside, they worked quickly but sloppily. Books were pulled from shelves and laid on the floor to muffle the sound of the main safe door being wrenched off its hinges. Twenty tin boxes of depositors' valuables were cracked open. In their haste, they left a chisel embedded in the inner safe wall and their workman's kit on the floor. They walked out with $2,747,700 -- $12,000 in cash and the rest in securities and certificates.

The robbery's irony was that most of what was stolen was worthless to the thieves. The certificates and securities could not be spent or cashed without detection. Shevlin gave $600 to an associate to hire a lawyer in Washington, D.C., to try to block a Congressional bill that would authorize replacement bonds for the stolen ones. The lawyer failed. A series of informants eventually led investigators to the gang members. Some were convicted; others bribed their way out. Officer John Nugent, who had been bribed to look the other way during the planning, was acquitted -- allegedly because he bribed a juror -- but was later convicted in Hoboken for highway robbery. Of the $2.7 million stolen, most was returned to the bank. Just $15,000 was never recovered. The investigation also revealed the full scope of Leslie's career, connecting him to nearly every major bank robbery in the United States over the previous decade -- all planned by a dead man whose body had been dumped in the woods five months before his greatest heist was executed.