Police were called to the Daventry International Rail Freight Terminal (DIRFT) on Thursday October 23 to reports of suspected illegal immigrants emerging from a lorry.
At around 2.30pm a lorry pulled into one of the depots on the site and when its container was opened 15 people jumped out.
The police were called and four were detained on the site - two Vietnamese people and two Iraqis.
A further five were discovered at one of the nearby wind farms, although there are no further details about them.
It is understood the sealed container had come across from France and it had first been opened at DIRFT.
Those captured were passed on to the UK Border Force.