A judicial source said the magistrate would decide by Tuesday morning whether to let her go or insist that she remains on the Mediterranean island of Sicily.
Rackete, a volunteer for German charity Sea-Watch, has become a symbol of divisions in Italy over the government's policy of closing ports to non-government rescue ships.
Interior Minister Matteo Salvini has called her a pirate, an outlaw and a people-smuggler while left-wing politicians have hailed her as an anti-populist heroine.
Rackete had been in international waters for more than two weeks, waiting for an invitation from Italy or another European Union (EU) state to accept her ship, which was carrying 41 African migrants.
"We need support to end the death of people at sea and it's outrageous that civilians ... filling a gap left by the authorities are criminalized for saving lives," said Giorgia Lunardi, a spokeswoman for the charity.
Germany called on Monday for her release.
Italian Prime Minister Giuseppe Conte told Germany's Chancellor Angela Merkel at an EU meeting that it was an independent judicial matter.
"Merkel asked me about the captain and I explained that in Italy executive power is separate from the judicial one ... the prime minister cannot step in. It's in the magistrates' hands," Conte told reporters in Brussels.
The Italian government accuses Rackete of endangering the lives of four policemen aboard a patrol boat that was squeezed against the quay as the Dutch-flagged Sea-Watch 3 forced its way into Lampedusa port early on Saturday.
She is also accused of resisting a warship order, which carries a maximum penalty of 10 years in jail, and of aiding people smugglers under recent laws introduced to deter the non-governmental rescue ships.
Rackete's boat was impounded and Sea-Watch could be fined up to 50,000 euros ($57,000).
Salvini, who is leader of the ruling right-wing League party, has taken a hard stance on immigration, closing off ports and accusing fellow European nations of leaving Italy alone to deal with new arrivals.
"I expect justice to deliver tough penalties to those who risked the life of Italian military personnel and who repeatedly rejected our laws," Salvini said in a statement on Monday.
He has said that if Rackete was released he would sign an expulsion order straightaway.
The 41 migratns were taken to a reception centre on Lampedusa, with Salvini later saying other European nations had agreed to take them in.
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