Compromised data was used to make unauthorised purchases in fewer than 400 cases
Hackers posted dozens of confidential documents on dark web
Highly targeted message sent to users in 92 countries
Cascade of failings allowed Chinese hackers to access government emails, says US review board
Includes Social Security numbers, passcodes and contact details
Possibly linked to a DDoS in February
The hackers claim to have stolen 550GB of confidential information