On the completeness of products of solutions to the Helmholtz equation

It is established that the family of pairwise products of solutions to the Helmholtz equation regular in a bounded domain $D \subset \mathbb{R}^3$, and fundamental solutions to this equation with singularities at points from a straight line ${\mathcal{L}} \subset \mathbb{R}^3$, ${\overline{D}} \cap {\mathcal{L}}=\emptyset$, is complete in $L_2(D)$.