Seni Lewis, a 23-year-old postgraduate from south London, died following prolonged restraint by police officers while he was an inpatient in the Bethlem Royal Hospital. It appears that he was held face down on the floor for a total of at least 40 minutes in the course of two successive episodes of restraint, altogether involving some 11 police officers.

A member of the public might think the obvious next step would be for the Independent Police Complaints Commission (IPCC), tasked with investigating the circumstances of Lewis's death, to speak to those officers and test their accounts in interview. Instead, the IPCC decided at the start of the investigation that there was no possibility any officer involved in his death could have committed either a criminal or disciplinary offence. By doing so, those investigators denied themselves the power to interview those officers under caution. The investigation ended without the officers' accounts having been tested in interview by the IPCC, a fact the Lewis family found baffling.

So you would think families like the Lewises would welcome the announcement from the Home Office that the IPCC has been granted new investigatory powers to compel police officers to attend interviews as witnesses. You would be wrong.

The IPCC claims that the failure of its investigators to interview police officers is because it does not have the power to compel them. Dame Anne Owers, the new head of the IPCC, has said that the IPCC "will be better able to discover and publicise the truth" once they are given broader powers to interview officers as witnesses and not as suspects. But this is entirely misleading. It should be viewed with strong scepticism in light of IPCC failures to exercise their existing powers to interview officers under caution.

The decision not to do so in Lewis's case was entirely a matter of choice for the IPCC. Under its existing powers, it could have chosen to interview those officers and interrogate their accounts if only it had had the courage to do so. The attempt by the IPCC to explain away its failure to interview officers due to a lack of power is misplaced: it is in fact simply due to of a lack of will.

For Lewis's family, these failures eroded their trust in the IPCC and its ability to conduct a genuinely independent investigation. Information submitted to the home affairs select committee from the charity Inquest confirms that the Lewis family is not alone: the IPCC is failing on a systemic basis to subject police officers who have been involved in deaths in custody to the rigour of an interview under caution.

Interviews under caution provide legal safeguards to officers whose conduct is in question and protect the integrity of any future criminal proceedings, however unlikely they may be. Plainly there are occasions where the circumstances of a death in custody do not imply the possibility of wrongdoing on the part of any officer. On those occasions, the new powers may serve to help investigators get a fuller picture of the circumstances of the death. Reassurance is needed from the IPCC that investigators will not be using these new powers to interview officers as witnesses when the facts say that they should be treated as suspects.

Lewis's family, like other families in their position, simply want a meaningful investigation into their loved one's death. The reluctance of IPCC investigators to subject the officers involved in Lewis's death to an interview under caution was deeply troubling for them. The fact that large numbers of IPCC investigators are former police officers inevitably fuels the concern.

IPCC investigators need to exercise their existing powers to interview officers lawfully and properly, alive to the possibility that those officers may have committed either criminal or disciplinary offences. Unless there is cultural change in that direction, new powers granted to IPCC investigators serve simply as an excuse for them to continue conducting lacklustre investigations, which are incapable of identifying responsibility where someone has died in police custody.