According to some interrogation, the shoe bomb idea/attack came as a result of either him or another terrorist wannabe on LY flights realizing that El Al wasn't conducting detailed inspections of passenger footwear on passengers' feet.

It sounded like El Al was his dry run of sort and that the LY experiences of no such detailed inspections with LY was what gave him and at least one or two others confidence to attempt to take what was learned from flying LY and apply it to going after a US carrier.

What was said was that LY placed undercover armed personnel near to him on an/the LY flight and could watch him that way. Not like it would do much good when he would be in a bathroom. Not like it addressed his being ticketed for multiple LY flights.