Former U.S. lawmaker and Green Party presidential candidate Cynthia McKinney and several other activists remained in an Israeli prison Thursday after refusing to sign a deportation form that they claim is self-incriminating.

In a press release from the Green Party, McKinney said the form states that the Spirit of Humanity, a Greek-flagged relief boat carrying 21 activists, medical supplies, cement, olive trees and children's toys en route to Gaza, was violating the Israeli blockade and trespassing the country's territorial waters.

"We were in international waters on a boat delivering humanitarian aid to people in Gaza when the Israeli Navy ships surrounded us and illegally threatened us, dismantled our navigation equipment, boarded and confiscated the ship," she said in a statement, adding that they were immediately taken into custody. 

"Immigration officials in Israel said they did not want to keep us, but we remain imprisoned," she said. 

"State Department and White House officials have not effected our release or taken a strong public stance to condemn the illegal actions of the Israeli Navy of enforcing a blockade of humanitarian assistance to the Palestinians of Gaza, a blockade that has been condemned by President Obama."

The Israeli military issued a statement Tuesday saying that the boat had attempted to break a blockade of Gaza and was forced to sail to an Israeli port after ignoring a radio message to stay out of waters around Gaza.

Israeli Foreign Ministry spokesman Yigal Palmor has said Israel was planning to free the crew and passengers.

"Nobody wants to keep them here," he said earlier this week.  "They will be released as soon as they are checked."

But so far, only two activists have been released -- both Israeli citizens. The others are still being held in Givon prison in Ramla, Israel. 

Israeli authorities will hold the remaining activists in the prison for three more days before the government decides to release them or continue to detain them, the Green Party release said.

While it was the second time in a year that McKinney's relief boat was seized by an Israeli naval ship, it's the first time that longtime supporter of Palestinians has been detained.