According to the passage, towards the end of the 1990s----.

Immigrants have long complained about US
employers who cheat or abuse them and threaten
to have them deported if they protest. Generally,
the problem has been confined to the lowest
rungs of the workforce, such as Mexican farm
hands who enter the country illegally. But
nowadays, the weak US economy has sparked an
outbreak of abusive treatment among the legions
of professional immigrant employees who flocked
to the US on perfectly valid visas during the late-
1990s boom. Usually, theirs are the cases of
employers who do not pay full salary or benefits.