Former prince Andrew Mountbatten-Windsor has been arrested on suspicion of misconduct in public office.
After almost 10 hours in custody, he was "released under investigation" on Thursday, local time.
The brother of King Charles has been under increasing scrutiny over his ties to the late convicted sex offender Jeffrey Epstein.
The king stripped Andrew of all his royal titles last October.
A batch of documents from the latest drop of the Epstein files unearthed emails between Andrew and the disgraced financier that prompted authorities to take a closer look.
Then on Thursday, Mr Mountbatten-Windsor's 66th birthday, UK police made their move.
Here is what we know about the arrest, the police investigation, and the reaction.
Why was Andrew arrested?
Police have not yet revealed all the details.
Thames Valley Police (TVP) said, as part of an investigation, they arrested a man in his 60s from Norfolk on suspicion of misconduct in public office and were carrying out searches at addresses in Berkshire and Norfolk.
Assistant Chief Constable Oliver Wright said that, following a thorough assessment, the police have opened an investigation into this allegation of misconduct in public office.
"It is important that we protect the integrity and objectivity of our investigation as we work with our partners to investigate this alleged offence," he said.
"We understand the significant public interest in this case, and we will provide updates at the appropriate time."
Mr Mountbatten-Windsor has consistently denied any wrongdoing and has not been charged.
The Crown Prosecution Service, which decides on whether a charge has the potential to lead to a successful prosecution, defines misconduct in public office as the "serious wilful abuse or neglect of the power or responsibilities of the public office held".
Following Mr Mountbatten-Windsor's release from police custody, TVP said its searches in Norfolk had concluded.
Police are continuing to search his former mansion on the Windsor Estate.
When and where was he arrested?
We don't know exactly what time he was arrested.
But at roughly 8am on Thursday morning, local time, unmarked vehicles were seen arriving at Wood Farm at Sandringham in Norfolk, where he has been living since leaving his home on the Windsor estate last October.
[ARREST LOCATION MAP]The unmarked vehicles are believed to have been police cars.
What were police investigating Andrew for?
We don't yet know the specifics of the police investigation that led to his arrest.
At least nine separate UK police forces have confirmed they are assessing reports appearing to link the former prince to Epstein.
Those police forces are based right across the UK, including Essex, south of Norfolk, where he was arrested; Bedfordshire; Norfolk; Wiltshire; the West Midlands; and even Scotland.
Thames Valley Police, which covers Windsor in Berkshire, where Mr Mountbatten-Windsor used to live, is the force that made Thursday's arrest.
Mr Mountbatten-Windsor served as a trade commissioner for about a decade from 2001.
On Wednesday, Surrey Police appealed for information relating to reports in the Epstein files, although their statement did not name any suspects.
Essex Police have also said they are looking into information in the Epstein files regarding flights in and out of a major airport based in the county.
"We are assessing information that has emerged in relation to private flights into and out of Stansted Airport following the publication of the US DoJ Epstein files," the statement read.
Is the arrest related to the Epstein files?
Police have not confirmed this; however, representatives from TVP — the force that arrested Mr Mountbatten-Windsor — met with Crown prosecutors last week to discuss allegations about the former royal stemming from the Epstein files.
For example, emails in the Epstein files suggested that Mr Mountbatten-Windsor sent Epstein a report outlining investment opportunities in Afghanistan.
The document, titled Helmand: High Value Commercial Opportunities for Foreign Investment, Brief for HRH Duke of York, was dated December 19, 2010.
He has not responded to those reports.
What has the royal family and the government said?
The king released a statement on Thursday, saying he had learned of his brother's arrest with the "deepest concern" and that he would cooperate with any investigation.
"What now follows is the full, fair and proper process by which this issue is investigated in the appropriate manner and by the appropriate authorities," he said.
"In this, as I have said before, they have our full and wholehearted support and co-operation.
"Let me state clearly: the law must take its course.
"Meanwhile, my family and I will continue in our duty and service to you all."
In the morning, before Andrew's arrest, British Prime Minister Sir Keir Starmer was asked whether the former prince should voluntarily speak to UK police regarding the Epstein allegations.
"I think that's a matter for the police," he said.
"They will conduct their own investigations, but one of the core principles in our system is that everybody is equal under the law and nobody is above the law, and it is really important that that is applied across the board."
