The road was closed all Wednesday afternoon and into the evening.
Mr Harrington also asked people not to speculate about the crash and the circumstances.
Roger Hirst, the police, fire and crime commissioner for Essex, said: “This is a tragic, terrible incident in which the life of a valued and respected officer has been lost.
“We pay tribute to this officer who was carrying out his duties keeping Essex people safe and secure.
“Our thoughts go out to his family, friends and colleagues.”
Earlier, a flag of the force’s crest was raised at half-mast at the Essex Police headquarters in Chelmsford.
