Top U.S. officials are denouncing Scotland's decision to free the only man convicted in the 1988 bombing of Pam Am Flight 103 over Lockerbie, Scotland.
White House counterterrorism adviser John Brennan called last August's release of Libyan Abdel Basset al-Megrahi, in his words, "unfortunate, inappropriate and wrong."
U.S. Secretary of State Hillary Clinton also issued a statement, saying the United States "continues to categorically disagree" with Scotland's decision.
Meanwhile, U.S. lawmakers said families of the victims will continue to suffer until Scottish and Libyan officials provide more details on why al-Megrahi was freed.
US Senator Robert Menendez told reporters in New York Friday that a "cloud of suspicion" still hangs over al-Megrahi's release.
The U.S. Senate is investigating whether British oil firm BP lobbied for al-Megrahi's freedom in order to save an oil deal with Libya.
Scotland released al-Megrahi last August, eight years into a life sentence for his role in the 1988 bombing that killed 270 people, including 189 Americans.
Scottish officials said they made the decision in good faith, based on medical experts who said al-Megrahi was dying of cancer and had just months to live.
Al-Megrahi returned to Libya to a hero's welcome and is still alive.
British authorities urged Libya not to celebrate the one-year anniversary, warning it would be insensitive and offensive to families of the bomber's victims.
Observers in Libya say the anniversary of al-Megrahi's release is passing quietly. Al-Megrahi's brother told the Reuters news agency that the Libyan government instructed the family to remain silent.
BP has acknowledged it discussed a prisoner-transfer agreement with government officials in London and Tripoli three years ago. BP says it was trying to protect a billion-dollar oil deal with the Libyan government, but that al-Megrahi was not part of the agreement.
Scottish government officials say they had no contact with BP before they decided to free al-Megrahi.
Some information for this report was provided by AP, AFP and Reuters.