Transcript for 4 in custody in alleged poison gas terror plot

We begin tonight with chilling new developments in an alleged terror plot. Police conducting raids across Sydney, arresting four men they say wanted to take down a commercial airplane with an improvised device that may have used toxic gas. Security stepped up at airport terminals across the country. Authorities there probing reports the suspect may have had direct contact with ISIS. And the department of homeland security keeping a close eye on the investigation. We begin with Jennifer Eccleston. Reporter: Tonight investigators scrambling to find evidence of an alleged plot to bring down an airliner. Heavily armed police, some in Haza hazmat suits, others in gas masks, raiding homes across Sydney, Australia. Four men arrested. We do believe it is islamic-inspired terrorism. Reporter: Officials believe the terror plan involved an improvised device, describing the plot as credible. Sources tell ABC news that American intelligence officials are closely monitoring the case, because of suspected communications between the alleged plotters and ISIS. Also tonight, chilling new details from the newspaper, "The Australian." The suspects constructing an explosive device using poisonous gas that would kill or immobilize everyone on the aircraft. ABC news has not verified this report. If Australian media reports are correct, this would be the first time we have seen any sort of terrorist act that involved gas on an airplane. Reporter: The department of homeland security are assisting partners overseas and have taken extraordinary steps to protect flights bound for the united States. Airports across Australia intensifying armed patrols, warning passengers of vigorous security. And Jennifer joins us live from London's heathrow airport. Jennifer, what do we know about the suspects in this case? Reporter: Well, Tom, the Australian newspaper reporting the four suspects were dual citizens of Australia and Lebanon, and all four were reportedly related. Tom? Jennifer he can he wielston with that terror investigation, thank

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