Viking coins estimated to be worth at least £500,000 and "of major historical significance" have been recovered by police.
The large number of coins and silver ingot were seized from properties in County Durham and Lancashire.
Durham Police said "a number of people have been arrested" on suspicion of dealing in culturally tainted objects.
It declined to confirm how many people were arrested or how they allegedly came to be in possession of the items.
The haul contains coins of Alfred the Great of Wessex and his less well-known contemporary Ceolwulf II of Mercia.
King Alfred inflicted a defeat on the Vikings in AD 878, and experts believe the coins belong to an undeclared hoard consistent with the location of the Viking army at that time.
'Shape British history'
Dr Gareth Williams, curator of early medieval coins and Viking collections at the British Museum, described it as a "nationally important hoard".
He said: "The coins I have seen so far add significantly to our understanding of the political history of England in the AD 870s.
"This is the period in which Alfred the Great was fighting the Vikings, but which also led to the creation of a unified kingdom of England under Alfred and his successors."
The coins and ingot were handed over to the museum after being seized earlier this month.
Det Insp Lee Gosling, of Durham Police, said: "It is not every day we get the chance to shape British history.
"It is the legal responsibility of the finder of any precious metal objects that are over 300 years old to report them to the local coroner as possible treasure under the terms of the Treasure Act 1996."