Theoretical studies indicate that interactions between the Earth's mantle and fluid core could produce a near 460-day, retrograde circular component of the nutation, often called the free-core nutation (FCN). Until now this phenomenon has never been observed. We use the 5.5 years of VLBI observations primarily collected under project IRIS to search for evidence of the FCN. The observations are consistent with an irregular excitation process, and model which assumes a step excitation in the FCN amplitude to about 2.0 milliseconds of arc in late 1985 fits the data well. Theoretical analysis appears to rule out the strong Mexican earthquake of September 19, 1985, as a cause of the excitation.