Future wireless communications systems are expected to coexist with different commercial applications over the same frequency bands. This gives rise to co-channel interference, which strongly affects the error-rate performance. In this work, an orthogonal frequency-division multiplexing system with bit-interleaved coded-modulation is considered and the problem of data detection in the presence of unknown narrowband interference (NBI) is addressed. In particular, an iterative detector that operates according to the expectation-maximization algorithm is derived under the assumption that the interference power on each subcarrier can be modeled as a random variable with an inverse gamma distribution. Computer simulations indicate that the proposed solution is remarkably robust to NBI and outperforms existing alternatives in severe interference environments.