Simultaneous fast measurements of electron density and temperature, Dα emission and Mirnov probe fluctuations in JT-60U during type I ELMs have been performed. Density fluctuations, measured with two vertical chords of the far infrared interferometer (FIR) at low and high field sides, have been found instrumental in determining spatial structure of type I ELMs (fluctuations localized mainly at the low field side) and duration of its MHD phase. Two different mechanisms for the initial phase of the interaction between the ELM-induced power and particle flows into the scrape-off layer (SOL) and the divertor target have been considered. They may explain the observed difference in the behaviour of Dα emission spikes at the inner and outer targets, which is related to the value of electron collisionality at the pedestal before the ELM. In ELMs with low pedestal collisionality, sharp fronts of Dα emission spikes at the inner and outer strike points and their simultaneous occurrence, coincident with the start of large-scale oscillations observed on the FIR channels, is observed. It is concluded that in such ELMs, plasma-wall interaction is initiated by a power flow conducted to the target through electron channel. In ELMs with high pedestal collisionality (but still of type I), slower evolution of fast signals was observed, with more gradual Dα spikes and clear time delays between outer and inner Dα channels, consistent with the mechanism of convective parallel energy fluxes with the ion sound speed proposed at JET.