Understanding the effects of nonequilibrium on strongly interacting quantum systems is a challenging problem in condensed matter physics. In dimensions greater than one, interacting electrons can often be understood within Fermi-liquid theory where low-energy excitations are weakly interacting quasiparticles. On the contrary, electrons in one dimension are known to form a strongly-correlated phase of matter called a Luttinger liquid (LL), whose low-energy excitations are collective density waves, or plasmons, of the electron gas. Here we show that spectroscopy of locally injected high-energy electrons can be used to probe energy relaxation in the presence of such strong correlations. For detection energies near the injection energy, the electron distribution is described by a power law whose exponent depends in a continuous way on the Luttinger parameter, and energy relaxation can be attributed to plasmon emission. For a chiral LL as realized at the edge of a fractional quantum Hall state, the distribution function grows linearly with the distance to the injection energy, independent of filling fraction.Over the last decade, experimental advances in nanostructure fabrication have brought a resurgence of interest in the LL model because of the possibility to test its peculiar predictions [1,2,3,4,5]. Defining signatures of a LL such as spin-charge separation [6,7], charge fractionalization [8,9], and the power-law suppression of the local electron tunneling density of states [10,11,12,13,14] have been experimentally verified. Recently, LLs driven far from equilibrium have begun to receive attention [15,16,17,18]. Studying these systems offers the possibility to characterize novel aspects of electron-electron interactions and to understand energy relaxation processes that have not been apparent in the above-mentioned equilibrium experiments.Here we consider a LL driven out of equilibrium by local injection of high-energy electrons, far away from any contacts, at a fixed energy. Their spectral properties are extracted at another spatial point some distance away by evaluating the average tunneling current from the LL into an empty resonant level with tunable energy. In this work, we consider both standard (non-chiral) and chiral LLs, which are realized at the edge of fractional quantum Hall systems [3,4,12,13,14].For the standard LL and for probe energies slightly below the injection energy, we find that the inelastic component of the current shows a power law behavior as a function of the difference between injection and detection energy, with an exponent that continuously evolves as the interaction parameter is varied. We develop a perturbative approach which shows how injected electrons can relax by emitting plasmons inside the wire.For a chiral LL at the edge of a fractional quantum Hall state from the Laughlin sequence, an essentially exact calculation of the tunneling current is possible. Here, the inelastic part of the electron current increases in a linear fashion as the probe energy is lowered from the inje...