Bulgaria has emerged as a transit point to enter central and western Europe through illicit means.
Bulgarian police arrested 15 alleged smugglers and detained hundreds of migrants following a nationwide raids.
The operation was sparked after authorities found 40 migrants, including children, inside a tanker truck specifically modified to transport people.
Some 300 illegal migrants with irregular paperwork were also detained in the 48-hour-long mission.
Smugglers had "extraordinarily inventive" means of transport and routes to take the migrants into Central and Western Europe, the country's Interior Ministry said.
The incident comes on the back of talks for Bulgaria's possible accession to Schengen.
The European Parliament wants to see both Bulgaria and Romania as part of the visa-free zone – a move that has been met with resistance from Austria in particular.
Bulgaria remains one of the few EU countries that are not in the Schengen area.
Although Bulgaria maintains a wire fence on its border with Turkey, it has seen a rise in migrants trying to cross the border ever since the Taliban seized power in Kabul in August 2021.
The influx is also fueled by the politics of its neighbour Turkey, according to Asylum Information Database (AIDA), with the number of asylum seekers almost doubling in 2022.
Human Rights Watch (HRW) accused the country of "brutally" pushing back migrants with European border security force Frontex also involved in its borders.
Bulgarian police were harsher in the absence of Frontex officers, but the "dehumanizing" act continued in their presence as well, according to the migrant testimonies cited by HRW.
Bulgarian prosecutors had charged six people over the deaths of 18 Afghan migrants in a similar incident earlier this year in February.
The bodies of suffocated Afghans were found in a vehicle transporting timber, where some 52 migrants were "packed like in a tin can" for several days.
The arrested were handed 15 years of prison sentences for involuntary manslaughter and running a criminal trafficking organization.