Send an email.

The UN human rights office's Sri Lanka accountability project collects information and evidence on serious human rights violations in Sri Lanka and lists ohchr-slaccountability@un.org on its project page. If you are a victim, witness or organisation with first-hand information or documents about the Easter Sunday 2019 attacks or how they were investigated, you can email it there; include the source, title, date and language of any material, and say whether you consent to it being shared. Do not send anything you have no direct knowledge of.