WINDHOEK, Namibia (AP) — Civil rights activists said Monday that Congo's official investigation into the death of a human rights campaigner looks like a cover-up.

In an open letter released Monday on the sidelines of a Southern African summit chaired by Congolese President Joseph Kabila, more than 70 civil rights groups called for an independent inquiry into Floribert Chebeya Bahizire's killing.

The body of Bahizire, head of Voix des Sans Voix, or Voice of the Voiceless, was found in his car June 2 in a suburb on the outskirts of Congo's capital, Kinshasa. Voix des Sans Voix, one of the largest rights groups in Congo, said he appeared to have been strangled.

Several police were arrested in connection to the death.

"The official investigation has achieved almost nothing and bears all the hallmarks of a cover-up rather than any real attempt to uncover the truth," said Hubert Tshiswaka, whose Open Society Initiative for Southern Africa was among the groups that signed the letter.

"Only an independent inquiry has any chance of bringing those responsible for these appalling crimes to court — something President Kabila has promised to do," Tshiswaka said in Windhoek, Namibia on the sidelines of the Southern African Development Community summit.

Congolese government officials in Windhoek for the summit were unavailable for comment.

Some observers have said Bahizire's death was part of an official strategy in Congo to silence opposition voices, while others have said it highlights chaos amid a struggle for power in the sprawling, troubled central African country.

For the past two decades, Bahizire's Voix des Sans Voix has documented human rights abuses across Congo, focusing on corruption in the military and foreign support for militias, according to the U.N.

The Southern African Development Community summit will run for two days, during which the rotating chairmanship will pass to Namibia.