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Given an edge-weighted graph and a set of known seed vertices, a network scientist often desires to understand the graph relationships to explain connections between the seed vertices. When the seed set is 3 or larger Steiner minimal tree - min-weight acyclic connected subgraph (of the input graph) that contains all the seed vertices - is an attractive generalization of shortest weighted paths. In general, computing a Steiner minimal tree is NP-hard, but several polynomial-time algorithms have been designed and proven to yield Steiner trees whose total weight is bounded within 2 times the Steiner minimal tree. In this paper, we present a parallel 2-approximation Steiner minimal tree algorithm and its MPI-based distributed implementation. In place of distance computation between all pairs of seed vertices, an expensive phase in many algorithms, our solution exploits Voronoi cell computation. Also, this approach has higher parallel efficiency than others that involve minimum spanning tree computation on the entire graph. Furthermore, our distributed design exploits asynchronous processing and a message prioritization scheme to accelerate convergence of distance computation, and harnesses both vertex and edge centric processing to offer fast time-to-solution. We demonstrate scalability and performance of our solution using real-world graphs with up to 128 billion edges and 512 compute nodes (8K processes). We compare our solution with the state-of-the-art exact Steiner minimal tree solver, SCIP-Jack, and two serial algorithms. Our solution comfortably outperforms these related works on graphs with 10s million edges and offers decent strong scaling - up to 90% efficient. We empirically show that, on average, the total distance of the Steiner tree identified by our solution is 1.0527 times greater than the Steiner minimal tree - well within the theoretical bound of less than equal to 2.