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We report here the observation of four impurity-induced lattice resonances in KI:Ag+. In addition to the well-known T1u symmetry resonant mode at 17.3 cm-1, we have observed two even-parity resonant modes of A1g and Eg symmetries at 25 and 16.35 cm-1, respectively, and a T1u symmetry gap mode at 86.2 cm-1. The two even-parity modes were detected using far-infrared spectroscopic techniques with the sample in an external electric field. The field induces a strong mixing of the modes of opposite parity and results in large field-induced shifts. The Eg symmetry resonance has also been seen in the Raman spectrum of KI:Ag+. Two weak and broad absorption lines at 29.8 and 44.4 cm-1 are attributed to the Eg+T1u and A1g+T1u combination bands, respectively. These results are interpreted in terms of a model in which the T1u symmetry resonant mode is anharmonically coupled to the even-parity resonances.