LONDON — The Russian double agent attacked by a military-grade nerve agent a month ago is no longer in critical condition, his doctors said Friday, joining his daughter in a recovery that could lead to the pair helping investigators solve the mystery of who, how and where they were poisoned.

Sergei Skirpal, 66, is “responding well to treatment, improving rapidly and is no longer in a critical condition,” said Christine Blanshard, medical director at the Salisbury district hospital.

The former spy’s 33-year-old daughter, Yulia Skripal, was also poisoned, but her condition had improved to stable. She is issuing statements and talking to relatives on the telephone.

The deadly nerve gas used in the poisoning of Yulia Skripal, above, and her father, Sergei Skripal, required officials to wear chemical suits as they investigated the crime in early March.

“As Yulia herself says, her strength is growing daily and she can look forward to the day when she is well enough to leave the hospital,” her doctor said.

After the doctor’s statement on the elder Skripal’s improved health was reported, the Russian Embassy to the United Kingdom tweeted, “Good news!”

The Russian government has denied having anything to do with the attack, insisting that Britain either rushed or bungled the investigation – or that British agents themselves may have poised the father and daughter to smear Russia, increase military spending, or distract their population from the failures of Brexit negotiations.

British military laboratory says the nerve agent was definitively from the Novichok class, a type designed and manufactured by the former Soviet Union and Russia. The lab, however, cannot definitely say where this patch was manufactured.

British police investigators say that the highest concentration of residual nerve agent was found at Skripal’s front door in the quiet town of Salisbury, down the road from the Stonehenge ruins.

Prime Minister Theresa May has condemned the attack as a reckless and hostile attack by Russia on British soil. She and her government say that in addition to forensic evidence, Russia is only suspect – as it has assassinated critics aboard in the past.