It isn’t often that a previously unknown opera by a major 19th-century composer comes to light. But such is the case with Franz Liszt’s unfinished “Sardanapalo,” based on a drama by Byron and given its U.S. premiere in a concert performance Saturday afternoon at the Library of Congress. David Trippett, the British musicologist and pianist responsible for the reconstruction of the first act of the opera, was joined by members of Washington National Opera’s Domingo-Cafritz Program and the Washington Master Chorale.

Liszt, most prophetic of the Romantic composers, paradigm of the virtuoso pianist, proto-modern conductor and promoter of musical talent wherever he found it, was involved with opera all his life. But until recently, his only opera was thought to be a childhood effort, composed when he was 13 and produced at the Paris Opera in 1825. Liszt nevertheless continued to entertain potential operatic projects well into adulthood, though none were completed.

One of these is based on the quasi-legendary sybaritic Assyrian monarch, Sardanapalo, who prefers feasting with his concubines to governing and conquest. When his subjects revolt and the walls of his Nineveh palace are breached by a flood of the Euphrates, Sardanapalo has his concubines, slaves and horses slain, and cast upon his own funeral pyre, a scene immortalized in Delacroix’s monumental 1827 canvas, “The Death of Sardanapalus.” Liszt commissioned an Italian libretto and worked on the opera between 1849 and 1852, when he became preoccupied with other projects.

For well over a century, scholars were aware of some 111 pages of Liszt’s sketches for Sardanapalo preserved in a Weimar archive, but considered them too fragmentary to be of practical value. It wasn’t until Trippett examined these sketches that it became clear the vocal parts were complete for the entire first act. The piano accompaniment, written in a kind of musical shorthand, also contained clear indications for orchestration.

So it wasn’t that Liszt’s Sardanapalo was “lost” so much as lying in plain sight, awaiting discovery.

Saturday’s program began with a fascinating lecture by Trippett, touching on the musical and linguistic complexities of preparing the score for performance. After intermission came a performance of the first act of the opera, ably directed by Trippett from the piano. Joshua Blue’s expressive tenor was a perfect match for the complexity of Sardanapalo’s character. Bass-baritone Timothy Bruno brought a blend of authority and agility to the role of Beleso, the high priest and royal adviser. In the role of the Greek slave Mirra, soprano Alexandria Shiner was poised and confident, though her uniformly stentorian delivery robbed the character of much of its vulnerability. Twelve women from the Washington Master Chorale, superbly prepared by Thomas Colohan, sang the extended luminous opening chorus with commitment and grace.

Hearing the mastery of Sardanapalo, its deft characterizations and emotive power, raises questions about what its effect might have been on opera had Liszt completed it. As it is, light is cast on a heretofore unsuspected facet of one of the 19th century’s great creative figures.

Alexandria Shiner plays the Greek slave Mirra. (Arielle Doneson Photography)