“We’ll leave a scar on America that will never heal!” soon-to-be gunman Eric Harris exclaims excitedly, contemplating the massacre that he and Dylan Klebold are about to carry out at Columbine High School. Klebold is capturing his friend’s gloating on a handheld video camera, and Harris’s hugely magnified face is splayed across a nearby wall, his eyes lit up with ecstatic anticipation.

The multimedia scene is one of several trenchant sequences in the effective, if sometimes leisurely, 1st Stage production of “Columbinus,” the 2005 docu­drama about the 1999 mass shooting at Columbine High School. Alex Levy and Juan Francisco Villa direct the staging, which coincides with the 20th anniversary of Harris and Klebold’s rampage in Littleton, Colo. (and closely follows the anniversary of the 2018 massacre at Marjory Stoneman Douglas High School in Parkland, Fla.). With mass shootings a regular occurrence in this country, and with evidence that some perpetrators in the years since 1999 have taken inspiration from Harris and Klebold, it is all too obvious that the scar of Columbine hasn’t healed.

As it drives home that point, this production benefits from the incisive performances of Rocky Nunzio and Patrick Joy in the lead roles. Nunzio’s Harris often radiates cheerful affability, a trait that contrasts eerily with revelations of his inner rage. Joy’s slumping posture and hangdog look make Klebold a vividly unnerving presence, but the actor also achieves pathos — for instance, in a semi-expressionistic scene in which his character rehearses “Romeo and Juliet” with a female classmate (the very good Alex Reeves) as other students voice his frightened thoughts.

The diligent, animated supporting cast includes Jennie Bissell as a devout Christian, Thais Menendez and Joe Mucciolo as popular kids with concealed vulnerabilities, as well as Brett Cassidy and Jonathan Del Palmer. The supporting actors also sometimes double as parents or counselors, who occasionally appear behind partitions on the fringes of Kathryn Kawecki’s school-stairwell set, the spatial separation emphasizing the loneliness and alienation of the teenage characters.

The loneliness and alienation sometimes feel a tad generic because of the way the play is written. Conceived by PJ Paparelli , who wrote the script with Stephen Karam (the authorship is officially credited to the United States Theatre Project), “Columbinus” draws on documentary evidence, as well as interviews with survivors and others affected by Columbine and comparable events. In scenes set in a fictional high school, the play’s first half often paints a broad picture of insecure teenagers coping with everyday humiliations such as bullying in a locker room. This is familiar territory, from pop culture and, for many, real life.

Directors Levy and Villa draw out some of these high school malaise sequences in a manner that slows down the production: A brooding montage set to the Verve’s 1997 pop hit “Bitter Sweet Symphony” is a case in point. The show’s second half, which focuses more directly on Columbine, is fresher, brisker and more propulsive.

Conor Mulligan’s lighting, rich in moody effects, adds to the emotional resonance. But the visuals are minimal at the play’s climax — an evocation of the massacre that unfurls largely in darkness pierced by flashes of aching light. Conscripting our imaginations, the sequence is as harrowing as it is stark.

Columbinus, by the United States Theatre Project. Written by Stephen Karam and PJ Paparelli; dramaturgy, Patricia Hersch; conceived by Paparelli. Directed by Alex Levy and Juan Francisco Villa. Costumes, Kelsey Hunt; sound, Kenny Neal; props, Cindy Landrum Jacobs; projections, Robbie Hayes and Patrick W. Lord. About 2 hours and 40 minutes. $15-$39. Through April 20 at 1st Stage, 1524 Spring Hill Rd., McLean. 703-854-1856. 1ststage.org.