Police officers yesterday described how they saw a paranoid schizophrenic stab to death one of their colleagues during a chase down a canal towpath.
Detective Constable Mick Swindells was fatally injured in the attack as he tried to arrest Glaister Earl Butler in Birmingham on May 21 last year.
Butler (49) was being pursued after earlier threatening to decapitate a council carpenter who had come to fix his fence.
West Midlands Police officers sent to help detain him told a Birmingham Crown Court jury how Butler repeatedly ignored their shouts to drop the large kitchen knife he was carrying.
Three officers also let off CS spray into his face. But it appeared to have no effect.
One officer said in a statement read to the court: "It was as if he had been sprayed with water."
Giving evidence as Det Con Swindells's widow, Carole, looked on from the public gallery, Det Con Gareth Knowles said he watched as his colleague again confronted Butler under the M6 at Spaghetti Junction.
Det Con Swindells was two to three feet behind Butler, had his extendable baton raised at shoulder height and shouted "stop, police".
"The defendant stopped very suddenly. He swung around with the knife in his right hand in a slashing motion," said Mr Knowles.
"The knife came across the front of Det Con Swindells and his reaction was to stop immediately and arched his back away from the knife and avoid the blow.
"I saw the defendant with the knife in his right hand deliberately punch out into the chest of Det Con Swindells. I saw the knife go into his upper body. I heard Det Con Swindells scream and fall backwards.
"The man (Butler) turned and ran away." Butler, of Long Acre, Nechells, continued running even after being hit in the back with a baton round from armed officers sent to the scene.
He was eventually arrested as the officers drew their sidearms but after being cautioned, he said: "What's this about? What have I done?"
Butler denies murdering the 44-year-old father-of-one, from Burton upon Trent, Staffordshire, but admits manslaughter due to diminished responsibility.
Earlier, the court heard Butler had been detained twice previously under the Mental Health Act.
The trial continues.