The Independent Police Complaints Commission is investigating a collision between a police van and a cyclist.
The British Transport Police van hit the 30-year-old male at about 1540 BST on 31 March in Dalston, east London.
He was taken to hospital with head injuries, where he remains in a serious but stable condition.
The IPCC will look at the speed of the police van, whether emergency equipment was in use and if the officers followed the relevant policies.
The investigation has found a number of witnesses to the incident.
House-to-house inquiries and a search for CCTV in the area have already been conducted.
An IPCC spokesman said: "The Independent Police Complaints Commission is managing an investigation into a collision between a British Transport Police van and a cyclist in north east London.
"The IPCC received a referral and that day began managing an investigation by officers from the Metropolitan Police's directorate of professional standards and collision investigation unit."