Police in Rwanda said they are questioning a prominent critic of President Paul Kagame on suspicion of "offences against state security".
Diane Rwigara was blocked from challenging Kagame in August's presidential election, and has spent the last month in and out of police custody or under house arrest, under investigation for alleged tax evasion and forgery.
Officers took Rwigara and her mother and sister back into custody at the weekend and have until Thursday to charge or release them, police said in a statement received Monday.
"During ongoing investigations, police uncovered credible evidence linking the trio to offences against state security," police spokesman Theos Badege said.
He said the Rwigaras had been detained for questioning on Saturday following their "consistent refusal to cooperate with the police and publicly revealing information that is, by law, supposed to be confidential."
Under Rwandan law police can detain suspects for up to five days. No decision has yet been taken on whether to charge the three.
Before her latest arrest Rwigara said the police action was punishment "for standing against oppression and speaking my mind."
Other critics and opposition figures have also been detained in recent weeks in what observers say is a post-election crackdown on dissent.
Kagame, who took control of Rwanda after ending the 1994 genocide, won the August 4 election with 99 percent of the vote and under the recently amended constitution is permitted to rule until 2034.