KINSHASA, Congo - A leading Congolese human rights activist killed this week had suffered a pattern of intimidation because of his work, a senior UN official said, as a rights group questioned whether police were already staging a cover-up.

The body of Floribert Chebeya Bahizire, head of Voix des Sans Voix, or Voice of the Voiceless, was found in his car Wednesday in a suburb of Congo's capital. The rights group, one of the largest in Congo, said he appeared to have been strangled.

"For more than 20 years, Chebeya Bahizire had survived many death threats, arrests, and ill treatment due to his work as a human rights defender. He believed in the cause of human rights and was not afraid to pursue it against all odds," Navi Pillay, the UN high commissioner for human rights, said Thursday.

UN Secretary-General Ban Ki-moon expressed shock at Chebeya's death, saying "his reputation as a champion of human rights earned him the respect and admiration of his compatriots and of the international community," UN deputy spokeswoman Marie Okabe said.

The secretary-general noted that Congo's interior minister on Thursday ordered the state security services to investigate Chebeya's death and stressed that the investigation "should be thorough, transparent and independent, with full respect for due process and rule of law," Okabe said at UN headquarters in New York.

The UN peacekeeping mission in Congo is prepared to assist such an investigation, if requested, she said.

Amnesty International had called on the government to launch an investigation into Chebeya's death.

"We are stunned and appalled by the suspicious death of such a prominent and respected human rights defender.... It seems he may have paid the ultimate price for his valuable work," said Veronique Aubert, deputy director of the group's Africa Program.

The rights group said he was last heard from Tuesday night, when he sent a text to a family member saying that he had just met with a senior police official and was headed home. Passers-by later found his body.

A group of Congolese rights groups said Thursday that they thought the inspector general of the National Police, John Numbi, with whom Chebeya met, was behind the death and said they would lodge a complaint in court.

Neither Numbi nor any of his aides immediately returned calls for comment Thursday, but a senior police official said an investigation into the crime had been opened.

Human Rights Watch has already questioned the integrity of any investigation, noting that family and UN representatives were initially denied and then given only limited access to the body.

"These irregularities indicate there may already be an attempt to cover up the truth," said Anneke Van Woudenberg, senior Africa researcher at Human Rights Watch.

The rights groups and the UN all said they were concerned by a growing trend of harassment of human rights activists in Congo. In 2005, a human rights activist was killed in his home in the country's east. Since then, at least three journalists have been killed, and Human Rights Watch said none of the investigations into the deaths has been satisfactory.

For the past two decades, Voix des Sans Voix has worked to document human rights abuses across Congo, focusing on corruption in the military and foreign support for militias, according to the UN.

Associated Press writer Frank Jordans contributed to this report from Geneva, and Edith M. Lederer contributed from the United Nations.