An Italian prosecutor says he has evidence some of the charities saving migrants in the Mediterranean Sea are colluding with people-smugglers.
Carmelo Zuccaro told La Stampa (in Italian) phone calls were being made from Libya to rescue vessels.
Organisations involved in rescue operations have rejected accusations of collusion, saying their only concern is to save lives.
Italy is the main route for migrants trying to reach Europe.
Almost 1,000 people are thought to have drowned in waters between Libya and Italy this year, according to the UN refugee agency.
Nearly 37,000 people have been rescued over the same period, a surge of more than 40% from last year, the figures say.
"We have evidence that there are direct contacts between certain NGOs [non-governmental organisations] and people traffickers in Libya," Mr Zuccaro is quoted as saying in La Stampa.
He said that telephone calls were being made from Libya and rescuers were shining lamps to direct smugglers' vessels and turning off transponders so boats could not be traced.
But he did not say he would open a criminal investigation.
Mr Zuccaro's comments come amid growing criticism of NGO activity in the Mediterranean, including one leading politician saying it was tantamount to providing a taxi service to Europe.
But Chris Catrambone, who co-founded the Migrant Offshore Aid Station NGO to rescue migrants, told Reuters news agency "more would die if we weren't there".
Correction 8 May 2017: The comment about a "taxi service" was incorrectly attributed to border agency Frontex in an earlier version of this story.