The Slocum was due to depart at 8 am, but was delayed by two families who, citing a premonition of disaster the night before, quickly disembarked the ship at the last minute. Around 9:40 am, the General Slocum began its fateful journey up the East River. Hundreds of children jammed the upper decks of the ship to take in the scenes of the urban panorama slowly passing them by. The Slocum band played German favorites. A little before 10 am, the ship neared the narrow passage of Hell Gate between Wards Island and Queens, which was known for its treacherous and unpredictable currents.
Around this time and unbeknownst to Captain William Van Schaick, a small fire had started in the ship’s forward hold. Though the specific cause of the fire remains a mystery, it is widely believed that a carelessly tossed match had sparked a pile of packing hay in the barrels of drinking glasses for the day’s picnic. As black smoke started to rise from below deck, some of the crew opened the hatch to investigate the cause. But when they opened the door, a rush of oxygen poured into the room and transformed the small spark into a raging fire. The crewmen quickly rushed to put out the ensuing blaze, but to their horror, they found that many of the ship’s firehoses were either rotten or leaky. To make matters worse, none of the crew had been trained in fire safety: they attempted to smother the flames with charcoal. Ten minutes later, the crew finally decided to notify Captain Van Schaick of the fire. By the time he learned about the fire, he later testified, it had become a “blaze that could not be conquered,” and any attempt to put it out was “like trying to put out hell itself." The fire began to spread up through the wooden structure of the ship toward the festivities taking place above deck.
When passengers noticed the encroaching inferno, panic ensued as fellow church members trampled one another in a dash to reach the stern of the ship. A reporter from The Chicago Tribune described the ensuing scene:
"It was a spectacle of horror beyond words to express—a great vessel all in flames, sweeping forward in the sunlight, within sight of the crowded city, while her helpless, screaming hundreds were roasted alive or swallowed up in the waves—women and children with their hair and clothing on fire; crazed mothers casting their babies overboard or leaping with them to certain death; wailing children and old men trampled under-foot or crowded over into the water—and the burning steamboat, her whistle roaring for assistance, speeding on for the shore of North Brother Island with a trail of ghastly faces and clutching hands in the tide behind her— grayhaired mothers and tender infants going down to death together."