Watch Alt-J Recruit Questlove for Winding 'In Cold Blood' on 'Fallon'

Roots drummer, horn section joined British indie-rock trio for live version of 'Relaxer' single

Alt-J unleashed the new single "In Cold Blood" on Tuesday's Tonight Show. The British indie-rock trio were joined by Roots drummer Questlove, who beefed up the drum attack, and a guest horn section that offered dramatic swells and soulful counter-melodies.

Singer-guitarist Joe Newman led the band through multiple rhythmic shifts and tempo changes, anchoring the unusual song structure with a sing-along chorus.

The track appears on Alt-J's upcoming third LP, Relaxer, out June 2nd. The eight-track album, which follows 2014's This Is All Yours, also features the equally sprawling "3WW." Last week, the trio released that's track's mysterious video, which features a funeral procession in the mountains of Mexico.

Alt-J will kick off a North American tour July 27th in Baltimore, Maryland. The trek runs throughout mid-November and includes spots at festivals like Panorama and Outside Lands.