Witnesses heard a dull thud before the top two floors of the building collapsed, raining debris onto the street in Vienna's 15th district shortly after 10 a.m. Saturday (0800 GMT; 4 a.m. EDT).
"There was a cloud of dust, and lots of people screaming and running," Ted Knops, a 32-year-old resident, told The Associated Press in a telephone interview. "It was like a science-fiction movie when something comes bursting through the wall."
Police spokeswoman Adina Mircioane said a 19-year-old man who was rescued from the rubble died on his way to a hospital.
"We are looking for a woman in her 50s who is still missing," Mircioane said, adding that the five other people had been treated for minor injuries.
The cause of the explosion was still being investigated, she said.
A video taken by Knops showed a vast plume of dust enveloping the street moments after the incident. The video was reviewed by the AP and along with Knops' description appeared consistent with the police.
