A new effort is currently underway to answer some of the outstanding questions surrounding the world's worst-ever chemical attack on civilians at Halabja.

Returning to the Kurdish town 25 years on from the 1988 attack, the BBC's world affairs editor John Simpson interviewed those in search of identifying the original chemicals for the mustard gas used and the European companies suspected of supplying Saddam Hussein's Iraq.

The mastermind behind the operation, Saddam's cousin Ali Hassan al-Majid, previously known as "Chemical Ali," was tried and executed in 2010.