On Tuesday, the media reported that classified government documents were found in Pence's private home, but it was not clear at the time what information they contained.
According to one of the sources, a number of documents were likely used to prepare Pence for meetings abroad, and they could have been overlooked while packing up his office.
The sources noted that these types of documents normally contain basic biographical information about foreign leaders, but sometimes they can contain more sensitive information.
The FBI is working with US intelligence agencies to assess the importance of the documents, a US official told the broadcaster.
Another source said that the documents contained "low level" classification markings. According to the source, there was no mention of documents marked SCI or SAP - the designations used to describe the most classified materials.
The US public has expressed dissatisfaction with the fact that the highest officials of the country do not comply with the rules for handling classified documents. In August 2022, a large collection of such materials was discovered at ex-President Donald Trump's Mar-a-Lago residence in Florida. In January, it was revealed that similar finds had been made in President Joe Biden former office at the Penn Biden Center that he used during his time as VP under Barack Obama, in Biden's family home in Delaware, as well as his garage and library.