GREENWOOD VILLAGE, Colo. -- Part of a high-end condominium building under construction near the Denver Tech Center collapsed Thursday morning, seriously injuring four workers.

The $140 million building, called the Meridian At The Landmark, is located at 5455 Landmark Place. That is just west of Interstate 25 and just north of Greenwood Plaza.

The collapse occurred at 9:35 a.m., when concrete that was being poured on the 14th floor caused part of the floor to collapse onto the floor below.

At least 20 fire engines and ambulances were dispatched to the location.

"We were just in a sales meeting, and we heard a lot of noise," said Bret Walker. "It sounded like a big dump truck kind of collapsing. We looked back and saw all of the debris just falling out to the side of the building. Just by the noise alone we knew it was something serious."

Several injured construction workers could be seen being loaded into ambulances next to the building. Early reports said at least four of the workers were severely injured. At least 12 workers were released from the hospital. Officials said the most serious injury among those who were released was a broken wrist.

One of the four workers who were seriously injured was released Thursday.

Two workers were at Littleton Adventist Hospital in good condition, said hospital spokeswoman Rachel Robinson. An unidentified 24-year-old man was in fair condition at Swedish Medical Center, said spokeswoman Julie Lonborg, but the nature of his injuries wasn't known.

The fourth person who was in serious condition when admitted to the hospital was treated and released by Thursday afternoon.

"Nothing really collapsed on top of them," said Dan Mulroney, operations chief for South Metro Fire. "The floor gave way. It wasn't hard cement, it was wet and when it bowed they fell through."

"One person was trapped underneath the forum and had to be removed," said Rick Palese with Everest Development.

A construction foreman said at least 300 people were working on the building at the time of the collapse.

The Occupational Safety and Health Administration sent investigators to the site to begin an investigation into what went wrong.

"We have to evaluate the building to determine what happened and why it happened and how to make it safe," said Howard Hardy with South Metro Fire Rescue.