WASHINGTON -- An American armored vehicle on Thursday barged unannounced into the wreckage of the Doctors Without Borders hospital in Afghanistan that the U.S. bombed earlier this month, reigniting tensions between Washington and the aid organization over the Oct. 3 attack.

The armored vehicle forced its way through the closed gate of the ruined hospital in Kunduz at 1:30 p.m. local time, Doctors Without Borders told The Huffington Post in a statement Friday. The organization said it received no prior notice that American officials would be visiting and only learned after the vehicle's arrival that it contained investigators planning to explore the wreckage. Doctors Without Borders said the move violated a commitment by U.S., NATO and Afghan investigative teams to inform Doctors Without Borders before taking any steps involving the aid organization's team or facilities.

The intrusion may have damaged evidence at the site that could help explain the bombing, the organization says.

"Their unannounced and forced entry damaged property, destroyed potential evidence and caused stress and fear for the team," said Tim Shenk, a press officer for Doctors Without Borders. The organization lost 12 staff members and 10 patients in the bombing.

Gen. John Campbell, the top U.S. commander in Afghanistan, has called the bombing a "mistake ," and President Barack Obama apologized to the president of Doctors Without Borders. The Pentagon, a joint U.S.-Afghan team and NATO are all investigating the causes of the incident. (Shenk declined to clarify on Friday which of these investigative teams had made commitments to provide notice before visiting the hospital site, but said members of all three teams were on the armored vehicle.)

But the incursion suggests that the government probes may be heavy-handed and ineffective, trampling on the aid organization's rights and, perhaps, on clues that remain at the site of the bombing. Photographs released Wednesday by Foreign Policy magazine showed how precarious the situation is within the ruined hospital, where charred human remains appear to lie on the dusty ground amid broken beds and ruined medical equipment.

The White House declined to comment Friday, instead deferring to the Pentagon, which did not respond to a request for comment.

One alternative to the Americans' already controversial approach presented itself this week when the International Humanitarian Fact-Finding Commission, an independent group created under the Geneva Conventions, said it had told the U.S. and Afghan governments it was willing to look into the incident. Neither government is party to the commission, however, and it's unlikely that an investigation will be opened.

Doctors Without Borders has called for an independent investigation since the incident occurred. Shenk noted Friday that the group's petition urging Obama to agree to an IHFFC probe garnered 50,000 signatures in 24 hours. The organization's push for an independent inquiry may gain traction now that it appears that the government investigations involve sending over armored vehicles unannounced.