Investigators have released a scientist detained at Miami International Airport after screeners spotted a metal canister in his luggage that looked like a pipe bomb, prompting an evacuation, an official said Friday.

No charges were filed against the 70-year-old man and he was allowed to continue his trip, a senior law enforcement official said. The official spoke on condition of anonymity because he wasn't authorized to release the information.

Neither the professor's name nor his itinerary has been released. The man is an American citizen and was "very co-operative," FBI agent Michael Leverock said at a news conference in Miami.

The metal canister that sparked concern was a legitimate experiment, said another government official, who also requested anonymity to discuss the ongoing investigation.

That official said the man has a prior arrest record related to biological material and is a professor at Ross University in Dominica on a teaching assignment in Saudi Arabia. The professor told law enforcement officials that the metal canister was used for medical testing, and the FBI found that it was used to transport dead bacteria samples, the official said.

Most of the airport was shut down Thursday night after officials found the canister in the scientist's bag. A Homeland Security spokesman said at first it looked like a pipe bomb, but no explosives were found.

A police bomb squad spent hours scouring the airport, passengers had to be evacuated from four of the airport's six concourses and airport roadways were closed down, police and airport officials said. They described the shutdown of the concourses as a public safety precaution.

The airport fully reopened just after 4 a.m. ET Friday before the first scheduled morning departures, which signalled the start of the peak Labour Day weekend.

"Everything's back to normal," airport spokesman Greg Chin said.