Double ionization following the absorption of a single photon is one of the most fundamental processes requiring interaction between electrons 1-3 . Information about this interaction is usually obtained by detecting emitted particles without access to real-time dynamics. Here, attosecond light pulses 4,5 , electron wave packet interferometry 6 and coincidence techniques 7 are combined to measure electron emission times in double ionization of xenon using single ionization as a clock, providing unique insight into the two-electron ejection mechanism. Access to many-particle dynamics in real time is of fundamental importance for understanding processes induced by electron correlation in atomic, molecular and more complex systems.The emergence of attosecond science (1 as = 10 −18 s) in the new millennium opened an exciting area of physics bringing the dynamics of electron wave functions into focus. The important goal of real-time visualization of the interplay between electrons and their role in molecular bonding now seems to be in reach. After a decade where attosecond light sources 4,5 were characterized and their potential demonstrated, the next phase will include the exploration of correlated electron dynamics in complex systems. A series of ground-breaking studies on single ionization (SI) in atoms using attosecond light pulses sheds light on the escaping electron and its interaction with the residual ion 6,8 , and the resulting coherent superposition of neutral bound states 9,10 . Double ionization (DI) by absorption of a single photon is an inherently more challenging phenomenon, both experimentally and theoretically 1-3 . The two-electron ejection can be understood only through interactions between electrons, and is usually discussed in terms of different mechanisms 11 . In the knockout mechanism, the electron excited by interaction with the light field (the photoelectron) collides with another electron on its way out, resulting in two emitted electrons. In the shake-off mechanism, orbital relaxation following the creation of a hole ionizes a second electron. Electron correlations may also lead to indirect DI processes via highly excited states of the singly-charged ion 12 . One-photon experimental investigations with the pair of electrons detected in coincidence can provide a fairly complete DI description without, however, following the dynamics of the electron correlation in real time. Multiphoton experimental investigations have been performed both on the femtosecond and attosecond timescales 13,14 , but DI in strong laser fields does not require electron correlation.In this work, we study DI of xenon in the near-threshold region using attosecond extreme ultraviolet (XUV) pulses for excitation and multi-electron coincidence techniques to disentangle SI and DI events. Using an interferometric technique with a weak infrared laser field, we demonstrate the existence of different ionization mechanisms and get new insight into the quantum dynamics of one-photon DI with evidence for inter-shell correlation e...