Two people were killed and six hurt Friday in the second mass shooting in the United States in as many days, with police still hunting the gunman in Orlando, Florida, emergency officials said.

Panicked workers reportedly barricaded themselves into their offices at the Gateway Center in central Orlando as SWAT teams combed through the 16-story building trying to track down suspected shooter 40-year-old Jason Rodriguez.

- The motive for the shooting, I do not know. I can confirm that he was a former employee at the business, what we've been told, but information is coming in, and it's fluid - said police spokeswoman Barbara Jones.

- It's my understanding at this time that it's one shooter - Jones said. - We're still seeking the suspect. - She did not give the name of his company.

A police helicopter hovered over the scene, as workers fled the scene and dozens of police cars locked down the area surrounding the center, television images showed.

Police first received a call that shots were being fired at the office building shortly after 11:30 am (1630 GMT). Reports suggested shots were still being heard at the center hours later.

- Eight total victims, two are dead - a fire department spokeswoman told AFP.

Bodies were found on the 12th and eighth floor of the building and the shooter had not been found, Deputy Chief Mike Droege of the Orlando Fire Department told the local Sentinel newspaper.

Four of the wounded were in a serious condition in hospital, emergency officials said.

- We were called to provide backup at around 12:00 noon to assist in the search for an active shooter - Jim Solomons, a spokesman from the Orange Country Sheriffs Department, told AFP.

The latest incident came with Americans still reeling from a rampage less than 24 hours earlier at a Texas military base.

President Barack Obama had only just ordered flags to be dropped to half-staff until Veterans Day on Wednesday when the latest shooting occurred.

The motive is not clear in Thursday's shooting which saw alleged shooter Major Nidal Malik Hasan, a 39-year-old specialist in combat stress, seriously wounded after killing 13 people and wounding 30 others.

Hasan, who had been fighting orders to deploy to Afghanistan, was under guard and on a ventilator after the rampage at Fort Hood, Texas, officials said.