HOUSTON -- A threatening note about a bomb was found on board a Southwest Airlines plane from Dallas to Houston Friday, prompting officials to isolate the aircraft and evacuate all the passengers, but no explosives were found, Local 2 reported.

The note was discovered on Southwest Airlines Flight 21 from Dallas.

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"A note was found on board that aircraft by a passenger and the note said something specifically to the effect that there is a bomb on the aircraft," said Al Tribble, with the Houston-area office of the FBI.

The 737 aircraft was isolated on runway 12 on the south end of Hobby Airport after it landed safely at about 1 p.m.

Nearly an hour after landing, police officers walked up the portable staircase to escort each passenger off the plane, one-by-one.

"They want to take account of who was on the plane and try to narrow down exactly who and where this note originated from," Tribble said.

The passengers were taken to an isolated gate, where they were questioned about what happened.

"The bottom line is --- we have to rule out that terrorism isn't a cause of this note," Tribble said.

K-9 units with the HPD bomb squad swept the plane and the luggage, according to Transportation Security Administration officials.

For family members, it was a scary and frustrating experience.

"They didn't give them a whole lot of information. I asked (my sister) how she was doing, and she said considering the circumstances, she was doing the best she could, so it sounded like, considering the situation, everyone was doing fairly well," Kyle Lednicky said.

Many family members were able to talk to their loved ones on cell phones.

"(My daughter-in-law said) 'There's been a possibility of a bomb onboard. So, they're not letting us deplane. They're not coming up to the gate.' I was able to reach her about three or four times on her cell phone," Sue Studley said. "I can imagine how frightened they were because they told them about it while they were still in the air. I think they should have gotten them away from the plane in case there was a bomb on board."

Some passengers were able to leave the airport at 5 p.m. after questioning.

"(They told us) that we'd have to be in an undisclosed area and they would give us more information," a passenger said. "I thought we'd be OK because they didn't get us off fast enough, so I thought then that we were alright."

City of Houston Aviation Department spokesman Roger Smith said no one was injured and no one has been taken into custody related to the note. He said passengers were checked by Southwest Airlines and airport security as they exited the plane.

"The actual crew will, in fact, be sequestered and they will be questioned by the TSA, along with the FBI on the origin of the note -- as much information as they can forward to local authorities as possible," said Rudy Lopez, with Southwest Airlines. "I cannot give specifics about the location of the note."

The plane was moved several hours after it landed and the runway was reopened.

Southwest officials said the flight was carrying 136 passengers and five crewmembers. The flight was scheduled to leave Dallas at 11:30 a.m., and then go on to Corpus Christi after stopping in Houston.