Use of 'transgender' Vs 'transsexual'.

Occasionally I see trans people refer to themselves as 'transsexual'. Presumably instead of 'transgender'. I always thought 'transsexual' was as best a dated term and at worst offensive, and had for all intents and purposes been replaced by 'transgender'. So why do some trans people use this term to describe themselves? Is there a subtle difference in meaning that makes one term more accurate than another for certain people?