One of the police officers present during the Hillsborough disaster kept a note of what happened in his pocketbook in what could be a "significant" new piece of evidence.
Watchdog the Independent Police Complaints Commission (IPCC) revealed that at least one officer made a note, against instructions, and that none of the previous inquiries into the tragedy had recovered any such notebooks.
Investigators have instructed South Yorkshire Police to search their archives for any relevant material.
The IPCC is overseeing an investigation into the aftermath of the disaster that claimed the lives of 96 Liverpool fans in 1989, in the biggest ever inquiry into police conduct in the UK.
IPCC deputy chairwoman Deborah Glass said in a monthly update on the inquiry: "It has emerged at least one of the officers has advised that they did complete an entry in their pocket notebook from the day of the disaster, contrary to instructions given.
"Having checked the archive records there is no trace of any pocket notebooks being recovered by any previous inquiry or the Hillsborough Independent Panel.
"As any such notebooks may prove to be a significant piece of evidence, we have instructed South Yorkshire Police to conduct a rigorous search of all their administrative storage areas. The IPCC will have independent oversight of this search."
Three interviews have been carried out in relation to claims that officers' statements were changed, and another 11 are scheduled.
Investigators are expecting to complete interviews on the topic by October.
Ms Glass said: "The initial interviewees have shown a willingness to engage with the investigation and give their accounts."