Players deported (clockwise from top left): Quinn, Van Dijk, Mapp, Hannay, Dalle Cort and Duggan
The Crusaders could be fined £60,000 over the visa irregularities that led to the UK Borders Agency deporting six of their Australia-born players.
In September the six were banished from the UK for 10 years.
A UKBA statement read: "Businesses have a legal responsibility to carry out the correct checks and the consequences are severe for those who do not."
The Rugby Football League, the sport's governing body, is to launch its own probe into the affair.
RFL boss Nigel Wood said: "We needed confirmation from the UKBA they had concluded their inquiries.
"That has only just been received so we are at liberty now to commence any rugby league specific inquiry, which we will be doing."
Jace van Dijk, Tony Duggan, Damien Quinn, Josh Hannay, Darren Mapp and Mark Dalle Cort were all found to have breached the conditions for their UK entry in a previous year and had their visas revoked.
Wood added: "We couldn't act beforehand because it might have been prejudicial to inquiries that were going on externally."
UKBA in Wales and the South West regional director Jane Farleigh said: "Having considered all the circumstances of this case, it was not considered appropriate to pursue a criminal prosecution against Crusaders Rugby League Club.
"However, the club has been served with a civil penalty notice for employing six illegal workers.
"The employers now face a fine of up to £60,000 unless they can prove to the agency that they carried out legally-required checks before employing workers from outside Europe.
"Businesses have a legal responsibility to carry out the correct checks and the consequences are severe for those who do not.
"The players involved have left the country and are now prohibited from working in the UK."