(CNN) -- The daughter of a prominent human rights activist says she has started a hunger strike in an effort to win the release of her father and other family members she believes are being detained by authorities in Bahrain.
Zainab Alkhawaja wrote an open letter to U.S. President Barack Obama, which she posted on her blog Monday.
"I am writing this letter to let you know, that if anything happens to my father, my husband, my uncle, my brother-in-law, or to me, I hold you just as responsible as the AlKhalifa regime," she wrote, referring to the ruling family of Bahrain. "Your support for this monarchy makes your government a partner in crime.
"I chose to write to you and not to my own government because the Alkhalifa regime has already proven that they do not care about our rights or our lives," she added.
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Alkhawaja's father, Abdulhadi Alkhawaja, is a former president of the Bahrain Center for Human Rights and a former protection coordinator with Front Line, an Irish-based human rights organization.
He was taken away after security agents stormed his daughter's home where he was staying in Manama, Bahrain, over the weekend, according to Front Line and to his daughter.
The Bahraini government did not immediately comment on his case.
"The way he was taken I don't know if he is OK or not. I know he was bleeding. I know he was unconscious," Alkhawaja's daughter told CNN. "Basically I don't want to be the next family to get a call saying please come and collect your relative's body and this is what's been happening in Bahrain.
"My father's only crime is that he is a human rights activist. His only crime is that he spoke up for people, that he demanded justice -- that he demanded democracy and for that, this is the price he's paying," she said.
Amnesty International released a statement Tuesday urging authorities in Bahrain to reveal the whereabouts and legal status of more than 400 opposition activists detained in recent weeks.
"We are increasingly concerned for the safety of these detainees, especially after reports of two further deaths in custody last Saturday," said Malcolm Smart, the group's director for the Middle East and North Africa.
U.S. State Department spokesman Mark Toner said he was aware of Alkhawaja's hunger strike and called on the Bahraini government to uphold the universal rights of its people.
Protests swept the strategically important island kingdom earlier this year as populations across the Arab world rose up against their rulers.
Bahrain, where the U.S. Navy anchors its Fifth Fleet, is a small, predominantly Shiite country governed by a Sunni royal family.
It called in Saudi Arabia and other regional troops to help end the protests last month.
"As a daughter and as a wife I refuse to stay silent while my father and husband are probably being tortured in Bahraini prisons. As a mother of a 1-year-old who wants her father and grandfather back, I must take a stand. I will not be helpless," wrote the younger Alkhawaja.
CNN's Salma Abdelaziz contributed to this report
