The fundamental bigroupoid $\Pi_2(X)$ of a space $X$ was independently described by Hardie, Kamps and Kieboom (paywall) and Stevenson (arXiv) around the year 2000. HKK cite Baez-Dolan's seminal HDA0, but the idea there is just very roughly sketched, and cited Grothendieck's Pursuing Stacks for inspiration. The idea goes back to a letter in February 1975 from Grothendieck to Breen describing the ideas later expanded in Pursuing Stacks:

Grothendieck's Feb 1975 letter to Breen http://imgur.com/4Sa2QDf

However, I would be surprised if the notion wasn't some kind of folklore. What other sources apart from those I've cited mention $\Pi_2(X)$?

Bénabou doesn't mention bigroupoids in his paper introducing bicategories, so a subsidiary question is: when were bigroupoids were first mentioned?

The fundamental bigroupoid $\Pi_2(X)$ of a space $X$ was independently described by Hardie, Kamps and Kieboom (paywall) and Stevenson (arXiv) around the year 2000. HKK cite Baez-Dolan's seminal HDA0, but the idea there is just very roughly sketched, and cited Grothendieck's Pursuing Stacks for inspiration. The idea goes back to a letter in February 1975 from Grothendieck to Breen describing the ideas later expanded in Pursuing Stacks:

However, I would be surprised if the notion wasn't some kind of folklore. What other sources apart from those I've cited mention $\Pi_2(X)$?

Bénabou doesn't mention bigroupoids in his paper introducing bicategories, so a subsidiary question is: when were bigroupoids were first mentioned?