The chair of the Sheku Bayoh public inquiry says its investigation will be "thorough" and "fearless".
Lord Bracadale, who was appointed in January earlier this year, has spoken in a video statement on the first day of the inquiry.
It will look at the facts surrounding the death of Mr Bayoh, which happened in Kirkcaldy in 2015.
The inquiry's chair has set out its background, purpose and details on how it will be carried out.
It isn't known how long it will take, but it's expected around 50,000 documents will be scrutinised.
Justice Secretary Humza Yousaf announced the inquiry in November last year.
Lord Bracadale said: "Today we launch an investigation that will be thorough and fearless.
"It has now been over five years since the death of Mr Bayoh and I, and my team, are conscious of the length of time this has hung over all involved, particularly the Bayoh family.
"We will work with determination and focus to ensure the work can be completed as quickly as possible.
"It is, however, at this stage impossible to say how long the Inquiry will take.
"It is only from today, the setting-up date of the Inquiry, that we are allowed by law to start ingathering the evidence.
"Preliminary discussions with some of the organisations involved lead us to believe that we will have in the region of 50,000 documents to scrutinise.
"This will clearly take some time for my team to get through."
In a statement by lawyer Aamer Anwar, on behalf of the Bayoh family and Mr Bayoh's partner Collette Bell, he said: "The family have welcomed the appointment of both assessors, Raju Bhatt and former Chief Constable Mike Fuller who bring a robust expertise across the full spectrum of policing, race and civil rights.
"Combining their expertise with that of respected Judge Lord Bracadale, the family have confidence that this inquiry will be fearless and impartial.
"But in the end the real test of this inquiry will not be the sympathy expressed for a family who have conducted themselves throughout with utmost dignity, but whether this country acts to ensure that real change takes place in an unaccountable, all powerful justice system."