Fewer than 25,000 pairs of those shoes were sold between 8/2018 and 1/2021. Check sales records in the D.C. Metro only, to begin. The number cannot possibly be that large (a few hundred??). Check the records for female purchasers first, ages 21-35. Cross-reference results with police records, and then expand outward.
Kitchen timers have sales records, too. They may have been purchased by a 3rd-party, but check hardware and kitchen supply store records in the region for 1-2 months before 1/5/21 and cross-reference those.
I'm sure the FBI knows how to do all of this, but it's shocking that they have no leads at this point.