Experimenting with murder

Upon opening his apartment door, two strangers suddenly appeared behind him.  Working quickly and in tandem, they completed their task – jabbing the apartment dweller with a syringe and injecting its contents – and then fled. The victim sought both help and accountability, calling a first-aid centre and chasing after his attackers. He did not get far in his chase, seemingly losing the ability to move after a short distance due to illness. He relayed the circumstances of the assault to the first-aid centre responder on the phone, gasped for a few seconds, and then provided his address. After this, he was non-responsive. Doctors arrived in ten minutes to discover the victim was dead. 

The ‘extremely short time between the injection to the victim and the confirmation of death’ was noted as one of the most striking features of this case by a team consisting of members from Sichuan University’s departments of forensic pathology and toxicological analysis, as well as the public security sub-bureau of Tianfu New Area in Chengdu, Sichuan, China.¹ The victim’s rapid demise, combined with his gasps on the phone and non-responsiveness that indicated asphyxia – later confirmed by postmortem examinations – pointed the team to an infamous poison. Cyanide. For alkali cyanides (such as sodium cyanide or potassium cyanide), absorbing 50–100mg from a single dose ‘may be followed by immediate collapse and cessation of respiration’. The pieces seemed to fit homicide by cyanide, yet the final piece – confirmatory tests – ruled out this fast-acting poison. 

A variety of specimens from the victim and the crime scene were collected for testing, which is typically targeted rather than a haphazard, look-for-everything affair. The fast action of the injected poison had set the initial testing target on cyanide. The next round of testing was guided by confessions. The two strangers that attacked the victim –described as ‘unwitting hired killers’ – confessed after arrest and pointed investigators to the ‘mastermind’.¹ Perhaps the true toxic potential of what was in the syringe was unknown to the attackers, though the mastermind knew and seemingly admitted all after being apprehended. In the syringe was an exceedingly rare agent in homicidal poisonings – tetrodotoxin. 

Tetrodotoxin is a neurotoxin found in select marine and terrestrial animal species. In humans, tetrodotoxin poisonings are typically due to ingesting tetrodotoxin-containing foodstuffs, most commonly pufferfish.² Dishes featuring pufferfish – also known as fugu – are prized but require highly specialised training to prepare for safe consumption, with even the heat of cooking no match for tetrodotoxin’s robustness. The onset, severity and progression of symptoms heavily depends on the dose, with initial symptoms typically a ‘pins and needles’ sensation (paresthesias), localised numbness, and possibly gastrointestinal issues. Higher grade symptoms include slurred speech, incoordination, paralysis and respiratory failure. 

A mild or low ingested dose sees symptoms begin 30 minutes to 6 hours later, with recovery after 24 hours, while severe or high doses ‘can lead to rapid onset of symptoms and progression to respiratory failure within 15 to 20 minutes’. Tetrodotoxin is often reported as being 100 times more toxic than cyanide, with tetrodotoxin having a lower minimal lethal dose. While cyanide has antidotes, there are no known antidotes for tetrodotoxin.

Extensive testing

The rapidity of the victim’s decline and the possible manifestation of symptoms seemed to fit with the use of tetrodotoxin. Analysts confirmed the use of tetrodotoxin via ultra-performance liquid chromatography–tandem mass spectrometry (UPLC–MS/MS) with the MS operating in multiple reaction monitoring mode for quantitative analysis, finding the toxin in cardiac blood samples but not urine samples. The absence of tetrodotoxin or its traces in urine is reasonable given the speed of death and the cessation of metabolic processes. 

The victim was injected with ‘a large amount of poison’, with confession records providing an idea of how much – ‘the suspect used organic solvent to dissolve three bottles of [tetrodotoxin] powder with a capacity of 1ml per bottle’.¹ The mastermind purchased the toxin via e-commerce, apparently under a false product name. Surprising to the Sichuan team was that the dose chosen for injection was not guesswork, but experimentally derived. Privately conducted mouse experiments and the victim’s estimated weight were used to calculate a lethal dose, with the final dose chosen such that it ‘significantly exceeded’ this calculated value. 

The number of documented tetrodotoxin homicidal poisonings appears to be countable on one hand – including the case detailed herein. Tetrodotoxin’s potential as a weapon is well known, but the practice of using it as one is blessedly rare.^3,4