The hole is 66 feet (20 metres) across and plunges nearly 100 feet (30 metres) deep.
Geologists said on Tuesday that the circular shape suggested a cave formation underneath, but what exactly caused the sinkhole was still a mystery.
"I can tell you what it's not: It's not a geological fault, and it's not the product of an earthquake," said David Monterroso, a geophysics engineer at the National Disaster Management Agency. "That's all we know. We're going to have to descend."
The sinkhole was formed Saturday and gulped down a clothing factory about three miles (two kilometres) from the site of a similar sinkhole three years ago.
Neighbours said it was a miracle no factory workers died.
"The boys were lucky," resident Honora Oliva said. "They left at six that afternoon, an hour before the earth opened up."
Neighbours said a weekend security guard also was spared because he had left to tend to his house, which flooded from heavy rain as Tropical Storm Agatha bore down on Central America. The storm has killed at least 180 people.
Some neighbours believe one or two people might have disappeared, but authorities said no deaths had been reported.
Crews were waiting for blueprints of the city's drainage system before investigating further.
The 2007 sinkhole killed three people and swallowed several homes in the same area. It was blamed on rain and an underground sewage flow, but it is too early to say whether those problems are to blame this time, said Monterroso, who also investigated the previous incident.
Police and soldiers stood guard around the sinkhole to prevent the curious from getting too close. Most people living nearby have moved out, fearful the hole will expand and swallow more homes.
"We're not going to wait," said one man who declined to give his name. "We've already rented elsewhere and we're leaving now."