An inquest, which concluded on Thursday, determined Andrzej Kusper, 38, died as a result of foreign body airway obstruction.
Mr Kusper became unresponsive and died around 8pm on September 4, 2021, at Leyton Custody Centre in east London where he was being held following his arrest.
Around 5.40pm that day, two Met officers were conducting proactive patrols in plain clothes on Leasowes Road, Leyton, when they stopped Mr Kusper.
One of the officers searched him on suspicion of drug possession. He was handcuffed to the front, and the officer found a small package containing a white substance and arrested him on suspicion of possessing a class A drug.
"When putting Mr Kusper into the back of a police van an officer noticed Mr Kusper putting his hand in his pocket," said the IOPC in a statement on Friday.
"The officer then searched Mr Kusper inside the police van but didn’t find anything.
"The officer did not notice Mr Kusper putting his hand to his mouth during this search. After the van doors were closed, the van CCTV footage showed that a bulge appeared in Mr Kusper’s cheek.
"The officer didn’t see this during the journey to Leyton Custody Centre.
"After arriving at the custody centre, officers noticed that Mr Kusper had something in his mouth and asked him to open it.
"When the officers saw something, they instructed him to spit it out.
"Officers took Mr Kusper down to the floor where he became unresponsive."
An ambulance was called for while officers carried out CPR, but Mr Kusper was pronounced dead at the custody suite shortly after 8pm.
An inquest that ended on Thursday found failings in both police searches of Mr Kusper probably caused or contributed to his death.
The inquest jury described both searches as “incomplete and unsatisfactory.”
They also found the police monitoring of Mr Kusper on his way to the custody suite in the police van probably caused or contributed to his death and said there was a “missed opportunity” to see the package in Mr Kusper’s mouth.
The jury concluded there were failings in the actions of officers in the custody suite, namely a lack of leadership and poor communication, and that this possibly caused or contributed to Mr Kusper's death, said the IOPC.
They also found Mr Kusper’s own actions contributed to his death.
The IOPC investigation looked into the police interaction with Mr Kusper prior to his death.
"At the conclusion of our investigation in November 2022, we decided that the officer who searched Mr Kusper inside the custody van should face a misconduct meeting for breaching the police standards of professional behaviour of duties and responsibilities," said the IOPC on Friday.
"This related to their failure to adequately search Mr Kusper following his arrest and for failing to properly monitor him during his transport to custody."
But the IOPC said a misconduct meeting held by the force decided there would not be a disciplinary outcome for the officer, who would instead go through a 'reflective practice review process' to consider opportunities for learning.
The IOPC also found two officers in the MPS’ Directorate of Professional Standards should go through the same process, for their mishandling of exhibits.
"One officer stored a water bottle in the same bag as Mr Kusper’s phones, which leaked and damaged the phones," said the IOPC. "The other officer incorrectly stored biological samples following the post-mortem examination which affected the ability to analyse them."
IOPC Regional Director Charmaine Arbouin said: “Our thoughts are with Andrzej Kusper’s family, loved ones, and everyone affected by his death.
“We conducted a detailed investigation, independent of the police, in order to establish the circumstances of this tragic incident.
“While it’s clear Mr Kusper placed the item in his mouth which he subsequently choked on, it was our view that the officer’s lack of attention in searching and monitoring Mr Kusper on the way to custody meant the item was not seen before he put it in his mouth.
“This incident shows the importance of carrying out thorough searches of detainees being taken to custody and actively monitoring those being transported to custody.
"As part of our investigation, we reviewed police body worn footage as well as CCTV footage from the police van and custody suite. We obtained and reviewed statements from the police officers involved and reviewed relevant police policies, training and guidance."
Following the conclusion of Mr Kusper's inquest, Met Police Commander Paul Trevers said on Friday: “I was hugely saddened to hear of Mr Kusper’s death. It is a tragedy and I cannot imagine the impact his loss has had on his friends and family. I hope the inquest has provided them with some answers.
“Now that the inquest has concluded, we will be writing to them to express our sincere condolences.
“As the coroner heard, we routinely train officers in how to keep people safe when they are arrested, in line with national guidance. However, we will carefully study the jury’s findings in relation to the search, and any recommendations from the coroner to consider what else we need to do.”
A Met spokesperson added: "The IOPC did not find any organisational learning about how the Met searches or transports prisoners, but they did identify some organisational learning around exhibits and training, and we have improved our practice in both of these areas as a result of these recommendations.
"We know it is important to get searches and transport right and, while we have made improvements, we know we can do better and will continue to work on this area for the safety of everyone."