The best conjunctive adverb to complete the text is "Still."
Here's the sentence with the chosen conjunctive adverb:
"In 1962, United States Army Flying Tiger Flight 739 disappeared over the Pacific Ocean. An extensive search party of eight ships, nearly fifty aircraft, and more than one thousand people was launched. Still, the plane was never found."
This indicates that despite the extensive search efforts, the outcome remained unfavorable.