Neil Basu says the "true horror" of the deadly poison was not knowing what to look for.
A former counter terrorism chief has described how he initially wondered if the poisoning of a former spy and his daughter could have been "an act of war".
Former Russian spy Sergei Skripal and his daughter Yulia were exposed to the deadly nerve agent Novichok in Salisbury in March 2018.
Neil Basu, who led the counter-terrorism investigation, said the "true horror" of the "colourless and odourless" poison was not knowing how to warn people or what to look for.
In an exclusive interview with the BBC's Salisbury Poisonings podcast, he said: "To leave that lying around anywhere on foreign soil is the most unbelievably reckless disregard for human life I've ever witnessed."