Anna Michalska of the Polish Border Guard told the Polish Radio News Agency IAR that "further steps in this case are carried out by the prosecutor's office."
Meanwhile, Belarusian authorities said on Sunday they discovered the body of an Iraqi woman near the border with Poland.
Earlier on Sunday, Polish Border Guard officers, together with firefighters, police, military and air ambulance services, conducted an hours-long operation to help eight immigrants – five men and three women – who got trapped in a swampy area near the Supraśl river.
"These people – a spokeswoman for the Podlasie border guard unit said – were exhausted and unable to get out of the swamp on their own."
Seven of those rescued were taken to the hospital, the border guard said.
In another case, near the village of Frącki, residents picking mushrooms came across three foreigners, one of whom was dead. The two other men, most likely from Iraq, were sent to hospitals in Suwałki and Augustów in the Podlasie province bordering Belarus.
Prime Minister Mateusz Morawiecki wrote on his Facebook account that Poland would look into "the possible link between these dramatic events and the provocative actions taking place on our (Polish ed.) eastern border."
Poland and its fellow EU members Lithuania, Latvia and Estonia have accused Belarus' strongman leader Alexander Lukashenko of organising a wave of illegal migrants seeking to enter the bloc, as part of what officials have called a "hybrid war."
At the beginning of September, Poland's president declared a state of emergency in parts of two regions along the country's eastern border with Belarus in a bid to stem the flow of migrants from countries such as Iraq and Afghanistan.
The state of emergency is expected to give authorities broader powers to monitor and control the movement of people.
Since the beginning of September, Poland's Border Guards have recorded over 3,800 attempts to illegally cross the Polish border from Belarus.