The Los Angeles Joint Terrorism Task Force is investigating an incident involving an unidentified man who spilled a vial of mercury inside a subway station just before Christmas and then disappeared after reporting it to authorities.
A statement released by the task force Wednesday evening shows a picture of the man taken from a frame in a surveillance video. The caption reads: "Wanted for questioning in connection with unexplained activity."
That activity can be seen in a copy of the surveillance video -- obtained exclusively by CNN -- which shows the Pershing Square subway station in Los Angeles late on the evening of December 22.
In it, the man crouches on the train platform and spills about five fluid ounces of mercury from a small bottle. He then goes to a Metropolitan Transit Authority call box to report what happened.
While the incident could be an accident, a counterterrorism expert told CNN it should set off warning bells.
"It doesn't make sense," said Ken Robinson, who used to work in intelligence for the Pentagon and has analyzed dozens of al Qaeda tapes for CNN. "He's got a heavy metal, and he's taking it into a subway. There's no good reason to do that. None."
Mercury found in thermometers is dangerous when swallowed, but spilling it would have no immediate toxic effect.
That's one reason the L.A. County Sheriff's Department, the lead agency in the investigation, believes the spill was just an accident and not terror-related. Also, the man who spilled it quickly called authorities to alert them.
"At this point we are relatively confident that it is not a credible threat," L.A. County Sheriff's Department spokesman Steve Whitmore said.
But a joint FBI and Department of Homeland Security intelligence bulletin released in 2005 warns that terrorists may make calls to test police reactions.
In the case of the spilled mercury, according to the HAZMAT cleanup report read to CNN, law enforcement did not respond for a full eight hours.
CNN analyst Pat D'Amuro, a former top FBI counterterrorism agent, says it's premature to rule out terror.
"I'm not saying that in this video these people are terrorists, but there's some very strange activity that needs to be identified here."
Shortly after the spill in late December, the sheriff's department sent out an alert, saying to be on the lookout for a man described as being "white or Middle Eastern," who is wanted in connection with a "possible act of terror."
In Thursday's statement, the terrorism task force said that although there is "no evidence to suggest these activities are terrorist-related, or even criminal in nature, we are seeking to resolve questions as to the reason and/or motivation of the mercury spill and why the unidentified male had the vial in his possession."