Online journalism requires stories to be lodged as fast as possible in competition with other news outlets, but investigative journalism pursues stories which are aimed at the people and places behind the headline news and often have a longer shelf life.
According to Lieb (2002 p. 43):
Some journalists make it a habit to follow up on stories they have covered in the past, and some news organizations have institutionalized these kinds of stories with regular follow-up features.Although the first assumption can be that investigative journalism is about finding scandals and conspiracies and award worthy stories, it is also about pursuing new angles on previously done stories, giving them an edge or showing a deeper and more colourful side to them.
Investigative journalism may even be aided by the internet, according to an article in The Guardian which points out how information and sources can be gathered a lot faster for an in depth piece using social media.
Lieb, T 2009, All the news: writing and reporting for convergent media, Pearson Education Inc, New York.