LONDON — British authorities investigating the poisoning of a former Russian spy, Sergei V. Skripal, and his daughter visited the graves of Mr. Skripal’s wife and son in the cathedral town of Salisbury, England, on Friday.

Dressed in large hazardous-material suits, the investigators began collecting evidence at Mr. Skripal’s house in the town and erected a blue forensic tent around the grave of the son. The police said that they had requested military assistance to “remove a number of vehicles and objects from the scene.”

Mr. Skripal’s wife, Lyudmila, 59, died in 2012 of uterine cancer, according to records from the National Health Service. His son Alexander, 43, died last year.