The Shanghai Cobra

The cobra venom method directly links back to Charlie Chan's previous case in Shanghai. Jan Van Horn, a dangerous criminal from 1937 Shanghai, had used similar poisoning techniques during his criminal career. The killer replicated this signature to draw Chan into the case and exploit his knowledge of the old investigation.

Chan's earlier memory of the Shanghai case is triggered immediately when he learns about the venom killings.

The killer concealed a spring-loaded poison needle inside the coin return mechanism of a jukebox located in a restaurant frequented by the bank employees. When a victim pressed the button to retrieve a returned coin, the needle sprang out and injected cobra venom through the skin. This made the murders appear to come from a cobra without any actual snake being present.

Chan notes early on that the victims died far from where anyone could have direct contact with a snake.

John Adams, who posed as the chief bank guard, was actually Jan Van Horn. After being badly burned during the Japanese bombing of Shanghai in 1937, he underwent plastic surgery which changed his appearance enough to assume a new identity. He escaped custody and lived under the name John Adams until unmasked at the end of the investigation.

Inspector Davis mentions that the criminal from the Shanghai case disappeared after plastic surgery, hinting at an altered appearance.

Mr. Jarvis, the owner of the chemical laboratory inside the bank, orchestrated the entire operation. He designed the poison-needle devices, including the lighter used to kill Detective Larkin, and directed the crimes from behind the scenes. The booth operator at the jukebox control room worked for him but had never met him face to face.

Chan notes that someone with chemical expertise was needed to engineer the poison delivery system.

Chan deliberately leaked a false rumor that the radium would be moved the following day to force the criminals to attempt their heist sooner than planned. This rush job would expose their methods and timing, allowing Chan and his team to catch them in the act. The plan worked, though Chan hadn't anticipated the criminals would use explosives.

The underground tunnel exploration reveals the criminals were preparing for a heist, making timing critical.

When the criminals attempted the heist, they used explosives to create a distraction in the tunnels beneath the bank. The blast caused a cave-in that trapped Charlie Chan, Tommy Chan, and Birmingham Brown inside the underground passages. Chan used ingenuity to tap into underground telephone lines and send a Morse code message to summon police rescue.

The discovery of the underground tunnels foreshadows the dangerous confrontation that occurs there.

The murder method using cobra venom was the direct connection. Jan Van Horn had used similar poisoning techniques during his criminal activities in Shanghai in 1937. When the Japanese bombed Shanghai, Van Horn was badly burned, escaped after plastic surgery, and later surfaced in the present-day bank under the alias John Adams to execute the radium heist.

Chan's memory of the Shanghai case is triggered as soon as he learns about the venom killings.

Detective Larkin was undercover as the bank's janitor to gather intelligence on the criminals. After learning too much about the operation, he was murdered by Mr. Jarvis using a cigarette lighter fitted with a poison needle. This was the same poison delivery method used in the earlier bank employee murders.

The timeline notes that Larkin's suspicious behavior foreshadows a broader conspiracy.

The fourth body discovered by Tommy Chan and Birmingham Brown in the underground tunnels was another victim of the same poison method. This confirmed that the killer was operating within the bank's hidden infrastructure, using the tunnels to move unseen and target victims who accessed specific locations.

The discovery of tunnels beneath the bank provides the means for unseen movement.

The booth operator ran the jukebox control room hidden behind a bookcase on the bank's second floor, but she worked exclusively through Mr. Jarvis's instructions without ever seeing him in person. This anonymity was intentional, allowing Jarvis to orchestrate the entire operation while remaining invisible to his own employees.

Chan notes early that the real mastermind hides behind a veil of anonymity.