An Alexandria woman on Thursday admitted she fatally stabbed and beat a 94-year-old retired optician who had once been her landlord, authorities said.

Marie M. Johnson, 45, pleaded guilty to second-degree murder and attempted arson in connection with the February slaying of Elmer “Joe” Roehrs, authorities said. During a hearing in Alexandria Circuit Court, Johnson also pleaded guilty to a separate burglary charge stemming from a Nov. 27 break-in at Roehrs’s home, on Holly Street in Del Ray, authorities said.

Johnson — who had once rented an apartment from Roehrs and his wife — was arrested hours after the 94-year-old’s body was discovered inside his home about 7 p.m. Feb. 13, although she was charged initially with only the earlier burglary, authorities said. Commonwealth’s Attorney S. Randolph Sengel said she was indicted in connection with the killing in May, and on Thursday pleaded guilty to the murder charge.

The crime was shocking both for where it occurred and for its unlikely, older victim. Roehrs, family members said, was a retired optician who exercised daily on the treadmill and spent his free time reading the newspaper, watching sports on television and fixing meals for his wife. He went by the name “Joe” because his first wife did not fancy the name Elmer, his stepson said in a previous interview.

Sengel said in an e-mailed statement that Roehrs was beaten and stabbed to death, and investigators determined that Johnson tried unsuccessfully to start a fire at the scene three times. He said the 45-year-old was caught on surveillance camera entering and leaving Roehrs’s home, and DNA evidence also linked her to the scene.

Marie Johnson (Courtesy Alexandria Police)

Johnson is scheduled to be sentenced Aug. 22. Public defender Melinda Douglas, her attorney, did not immediately return messages seeking comment.