Packet sniffing wi-Fi-hacking neighbor (

I read with interest this CNN article

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titled: "Wi-Fi-hacking neighbor sentenced to 18 years".

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Can someone further explain this sentence in that article?

"With Kostolnik's permission, they installed a packet sniffer on his network to try and get to the bottom of the incidents ... A forensics computer investigator working for Kostolnik's law firm examined the packet logs... In the data surrounding the threatening traffic, they found traffic containing Ardolf's name and Comcast account .

Two fundamental questions:

What packet sniffer would I install (Ubuntu & WRT54G) if I were to look for snooping neighbors ... and .. .
Why would a connection to the neighbor's router also leave the perpetrator's actual name and comcast account information?

Thanks for any advice as this both scares and intrigues me.