The morning after a five-alarm Mission Bay fire gutted a wood 360-unit apartment on Fourth Street, Mayor Ed Lee says "we're very lucky" that the fire didn't spread or that more people weren't injured (one firefighter had his face burned while battling the blaze.)

While what caused the fire remains under investigation, The Chronicle notes:

"It appears that the fire started above ground," Lee said. "They were doing some welding. We don't know what occurred there. I'll be very patient to wait until the investigation is concluded by our Fire Department and by the contractors themselves."

Most of the workers had left for the day before the blaze began, and investigators suspect that the fire could have smoldered for some time before erupting, said Fire Chief Joanne Hayes-White.

Evacuated residents in nearby buildings are slowly being let back in today. As for the structure itself, it's still in a precarious place.

Above all else, the fire raises concerns over the quality of construction in Mission Bay. The burgeoning neighborhood has always been a bit of a question mark, both aesthetically and structurally, especially when compared to other San Francisco neighborhoods. John King has more:

The San Francisco-based developer purchased two blocks totaling 3.7 acres in 2011 for $41 million. When construction began last year on the 1.8-acre block that burned Tuesday, company officials put the estimated total development cost at $227 million. At least four other residential projects are at various stages of construction within a stone's throw of the BRE site, together containing nearly 900 units.

Beyond the damage to the project dubbed MB360, there's the damage to the retail district planned along Fourth Street.

The ground floor of every residential building there is reserved for retail space. The idea is that the strip will become the commercial heart of the neighborhood - the 21st century equivalent to Chestnut Street in the Marina.

Even though other residential buildings did not sustain damage in Tuesday's fire, the presence of a burned-out block is bound to slow Fourth Street's emergence as a destination.

The area Fourth Street/China Basin are remains closed off as of this morning.

Previously: Breaking: 5-Alarm Fire Burning In Mission Bay Construction Site [Updates]