[MRMEAN]

Poultry plant officials said they had no way of knowing the workers were illegal, since the workers had provided identification and were hired through the state’s employment security division.

I daresay the plant management knew. And so did the state's employment security division. And if ICE wanted to look into it, they would know that they knew. But...

[TomGuy]

I cannot understand how these 'illegals' keep 'slipping through' the system [assuming someone/many aren't looking the other way].

Being a WASP, every time I have been offered a job in the last 30 years, I've had to provide 2 forms of identification, one of which HAD to be a photo ID.

[LNewman]

From the horse's mouth, so to speak ... This from Meat News

"With immigrant workers comprising a large percentage of the overall workforce of the meat and poultry industries in the USA, immigration reform is of intense interest to processing plant managers. In total, an estimated 11 million workers in the entire U.S. manufacturing and service sectors are estimated to be in the country illegally."