In an interview with Italian daily La Stampa, Sicily-based prosecutor Carmelo Zuccaro made his most specific claims yet over NGO activities off Libya, which the EU border agency Frontex recently described as tantamount to providing a "taxi" service to Europe.
Non-governmental organizations (NGOs) active in the rescue effort include long-established groups such as Doctors without Borders and Save the Children, and smaller, newer operations such as the Malta-based MOAS.
They have all dismissed suggestions of de facto collusion with smugglers as a baseless slur on volunteer crews whose only mission is to save lives in the absence of EU governments acting effectively to do so.
Over 1,000 migrants are feared to have died in waters between Libya and Italy so far this year, according to the UN refugee agency. Nearly 37,000 have been rescued and brought to Italy.
"We have evidence that there are direct contacts between certain NGOs and people traffickers in Libya," Zuccaro was quoted as saying by La Stampa.
"We do not yet know if and how we could use this evidence in court, but we are quite certain about what we say; telephone calls from Libya to certain NGOs, lamps that illuminate the route to these organizations' boats, boats that suddenly turn off their (locating) transponders, are ascertained facts."
Libya deal in doubt
Zuccaro is the head of a five-strong pool of prosecutors investigating criminal aspects of the migrant crisis, from trafficking to illegal exploitation of migrants on Italian farms and via prostitution to rackets in the provision of reception facilities.
La Stampa reported that prosecutors were looking into whether some of the newly-established NGOs may be financed by the traffickers as a way of making it easier to guarantee their human cargoes get to Italy.
The organizations involved have all dismissed the charges against them. They fear they are being targeted by a smear campaign designed to get them out of the way.
One group, SOS Mediterranee, told AFP last week it had "never, not once" been put in touch with a migrant boat via smugglers.
Under an EU-backed strategy, Italy is currently trying to beef up Libya's coastguard in the hope more boats can be prevented from getting out of Libyan territorial waters and the migrants returned to holding camps in the troubled country.
The strategy has been described by rights groups as a breach of Europe's obligations under international refugee conventions.
And it has so far made little headway towards closing down the Libya-Italy migrant route.
Rome said on Friday it would be providing Libya with ten new coastguard boats but a cooperation deal covering holding camps and repatriations is in limbo after it was suspended by Libya's Court of Appeal.
Food ban lifted
The number of people leaving Libya in the hope of starting a new life in Europe is up nearly 50 percent this year compared with the opening months of 2016.
With most departures coming in the warm summer months, the trend points to around 250,000 people arriving over the course of 2017 - a forecast Zuccaro described as "an under-estimate".
Some 500,000 migrants were registered in Italy in the three years spanning 2014-16.
And pressure on the country's reception facilities has increased in the last year as a result of neighbouring countries tightening border controls, making it harder for migrants to move further north.
Tensions at bottleneck border points were underlined when Ventimiglia, a town on the Riviera border with France, issued an order banning locals from distributing food to migrants.
The order, similar to one issued in Calais in northern France, was withdrawn on Sunday in a move welcomed by aid groups who hope it will have a bearing on a court case against French activist Felix Croft.
Prosecutors have asked for a prison term and 50,000 euros fine for Croft, 28, for trying to help a Sudanese family from Darfur to cross the border into France in July 2016.
"This is excellent news. You cannot use the law to persecute solidarity, however it is expressed," said Patrizio Gonnella, president of Antigone, a civil rights group. Croft is due to learn his fate on Thursday.
By Angus MacKinnon
READ ALSO: Italy unveils plan to better integrate fewer migrants
Photo: Andreas Solaro/AFP