Although the letters were signed by 'Saud Abdulaziz Al-Saud', the report said that these were actually prepared and signed by a man named Mohammad Abdullah Al Koman.
Mohammad Abdullah claimed he was a representative of Prince Abdulaziz Majid Al-Saud.
"Mohammad Abdullah told investigators that he was the person who prepared the letters and signed all of them on behalf of the prince," said the report, adding that he could not provide more information regarding the billions transferred to Najib because it involved "the interests of the Saudi royal family."
"In other words, Mohammad Abdullah told Malaysian investigators that they had to take his word that it came from the Saudi royalty.
"Sources say after the meeting with Mohammad Abdullah, investigators concluded that 'it was not safe to confirm the money from Blackstone (Asia Real Estate Partners) and Tanore (Finance Corporation) were contributions from Saudi Arabia, because it was still unclear and there was a lack of supporting documents'."
Mohammad Abdullah also claimed that Eric Tan Kim Loong, the beneficiary owner of both Blackstone and Tanore, was a "trusted associate."
Tan has been described as a close associate of businessperson Low Taek Jho, a key figure in the 1MDB scandal. Low is also believed to have used Tan's name as an alias.
The US Department of Justice in its civil forfeiture suit filings had established that Blackstone and Tanore's funds originated from 1MDB.
The report said in total, there were three similarly worded letters in the name of 'Saud Abdulaziz Al-Saud.'
Each preceded a series of deposits to Najib' bank account, which is detailed here.
On Friday, police announced that they have established at least US$972 million entered Najib's bank account when he was prime minister.
In Aug 2015, Umno president Ahmad Zahid Hamidi, the deputy prime minister at the time, claimed he had met the "chief investment officer and wealth trustee" of the Arab family that Najib purportedly claimed donated the money to him.
Following the installation of a new government after BN's defeat in the May 9 general election, Zahid was questioned by the MACC in July about the meeting.