Nel Apgar was a Tanugan scientist working on Krieger waves who died in an explosion that Riker was accused of causing.
Dr. Nel Apgar was a Tanugan scientist working on a new energy source called Krieger waves aboard a research station in orbit of Tanuga IV in A Matter of Perspective. Riker and La Forge visited to check his progress, and the station exploded just after Riker beamed away, killing Apgar.
Apgar's wife Manua and his assistant Tayna gave accounts suggesting Riker had a motive. A holodeck reconstruction finally showed that Apgar was secretly building a weapon to sell, had tried to kill Riker, and had died when his own beam was reflected back by the transporter. Mark Margolis played him.