The officer was stabbed outside a pub in Bow, east London on November 22.
A 20-year-old man was arrested on Tuesday, 29 November in Bedfordshire on suspicion of assisting an offender - he has been bailed pending further enquiries to a date in mid-January.
Two 16 year olds have already been arrested on suspicion of attempted murder and remain in custody.
A 17 year old youth arrested on suspicion of murder has been remanded on bail.
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Two people have been charged as part of theinvestigation.
Andrew Beadie, 20 of Claremont Road, Basildon and Janine Morris, 24 of Rainhill Way, Bow were both charged on Saturday, 26 November with attempted murder and two counts of affray.
Morris was also charged with assisting an offender.
They both appeared in custody at Thames Magistrates’ Court on Monday, 28 November and were remanded in custody to appear at the Old Bailey on 4 January 2017.
The officer remains in hospital with non life-threatening injuries.