BL Lac objects are an extreme type of active galactic nuclei (AGNs) that belong to the largest population of $gamma$-ray sources: blazars. This class of AGNs shows a double-bumped spectral energy distribution that is commonly described in terms of a synchrotron self-Compton (SSC) emission process, whereas the low-energy component that dominates their emission between the infrared and the X-ray band is tightly connected to the high-energy component that peaks in the $gamma$-rays. Two strong connections that link radio and mid-infrared emission of blazars to the emission in the $gamma$-ray band are well established. They constitute the basis for associating $gamma$-ray sources with their low-energy counterparts. We searched for a possible link between X-ray and $gamma$-ray emissions for the subclass of BL Lacs using all archival Swift/XRT observations combined with Fermi data for a selected sample of 351 sources. Analyzing $sim$2400 ks of Swift/XRT observations that were carried out until December 2018, we discovered that above the $gamma$-ray flux threshold $F_{gamma}approx3times10^{-12},rm{erg},rm{cm}^{-2},rm{s}^{-1}$, 96% of all emph{Fermi} BL Lacs have an X-ray counterpart that is detected with signal-to-noise ratio higher than 3. We did not find any correlation or clear trend between X-ray and $gamma$-ray fluxes and/or spectral shapes, but we discovered a correlation between the X-ray flux and the mid-infrared color. Finally, we discuss on a possible interpretation of our results in the SSC framework.
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