Various approaches have been demonstrated for the automatic interpretation of crystallographic data in terms of atomic models. The use of a masked Fourier-based search function has some benefits for this task. The application and optimization of this procedure is discussed in detail. The search function also acquires a statistical significance when used with an appropriate electron-density target and weighting, giving rise to improved results at low resolutions. Methods are discussed for building a library of protein fragments suitable for use with this procedure. These methods are demonstrated with the construction of a statistical target for the identification of short helical fragments in the electron density.