LONDON — Medical workers in Britain who treated the former Russian spy Sergei V. Skripal and his daughter, Yulia, initially feared that they would be inundated with civilian casualties from a lethal, Soviet-developed nerve agent and that their colleagues had been exposed, they have told the BBC.

The accounts suggest that the British authorities were deeply apprehensive about the possibility of a larger health crisis in the hours after the Skripals were found unconscious on March 4, in what was initially believed to be an opioid overdose.

That evening, after hearing that Mr. Skripal had been a Russian spy, medical workers set aside that theory and began to catalog symptoms of poisoning by an organophosphate, or nerve agent.

“There was a real concern as to how big this could get,” Lorna Wilkinson, the nursing director at Salisbury District Hospital, said in an interview with the BBC Two program “Newsnight” that will be broadcast on Tuesday. “Have we just gone from having two index patients having something that could become all-consuming and involve many casualties? Because we really didn’t know at that point.”