Boston, June 4 (RHC)-- In the U.S. city of Boston, prosecutors say a man shot dead by law enforcement agents in Boston this week was involved in a plot to behead police officers.

Agents had been monitoring 26-year-old Usaama Rahim as part of a terrorism probe when they say they overheard him talking about beheading an officer. Rahim was shot dead after allegedly lunging at two officers with a knife.

On Wednesday, a relative of Rahim’s identified as David Wright was arrested and charged with conspiracy. Anonymous officials say Rahim became radicalized by online jihadist sites and was inspired by the Islamic State.

Briefing reporters, Boston Police Commissioner William Evans said he had been under 24-hour surveillance, saying that "the threat was severe enough that we had to approach him."