Officials sequestered a Los Angeles metro bus after a male passenger wearing a safety mask claimed to have Ebola. (Reuters)

A man wearing a surgical mask and a woman got onto a bus in Los Angeles Monday afternoon. He proclaimed, "I have Ebola!" Moments later, he threw the mask on the ground, and they both got off the bus.

Now, the FBI is involved in trying to track down the man, with an investigation being treated as a possible terrorist or criminal threat, according to Los Angeles Metro officials.

"Assuming this is a hoax, if people think it's funny to do something like this, they're not going to think it's funny when they are arrested and prosecuted," Metro spokesman Marc Littman said. "It's very disruptive to people's lives and costs taxpayers thousands of dollars."

There are no known cases of Ebola in California, and the virus is not airborne. Still, transit officials aren't taking the incident lightly. The driver immediately called central dispatch and offloaded passengers before he drove the vehicle to the bus yard, Littman said. The bus has been taken out of service, and the driver -- a 25-year veteran of Metro -- asked to be taken to the hospital to be checked out.

The driver is showing no signs of illness but won't be back at work for another two to three weeks, Littman said.

This is the second high-profile incident in less than a week where someone yelled about Ebola and caused all sorts of mayhem. A passenger on a US Airways flight from Philadelphia reportedly said Wednesday, "I have Ebola. You all are screwed" and yelled, "I've been to Africa" as the plane was about to land in the Dominican Republic. That prompted a hazmat crew to board the plane and escort the man off, who was heard saying, "I was just kidding."

A passenger reportedly said, "I have Ebola," after sneezing on a U.S. Airways flight from Philadelphia to Punta Cana, Dominican Republic, on Wednesday. The flight crew then alerted authorities (Patrick Narvaez)

Kidding around or not, such public declarations kick off a massive, and costly, response by law enforcement and public health officials. Littman said the Los Angeles incident has likely already cost thousands of dollars. A hazmat crew has to clean the bus before it's put back into service. The FBI is looking at the bus's surveillance video. "We definitely want this person," Littman said.

He also said transportation systems, from buses to planes, are entering "uncharted territory."

As Eugene Volokh writes, such shouting "is pretty clearly constitutionally unprotected, because it’s a knowing falsehood that has the potential to cause direct and substantial harm.” But it’s unclear what California statute could be used to punish the man for yelling “I have Ebola!”

Still, if you're wanting to make an Ebola joke, be very, very, very careful. You could end up paying a really heavy price for it.

Related: Americans are worried about Ebola, want travel restrictions