Buhari returned in March from a two-month medical absence in the U.K.
"My concern is whether we have cut and past a framework that doesn't work for Africa," says Amina Mohammed.
Despite help from the U.S., two successive Nigerian presidents have failed to liberate the vast majority of girls captured by Boko Haram.
The watchdog suspects the funds result from "unlawful activity."
If you have long hair and work for Nigeria's road safety corps, you might not be safe for long.
Nigeria has a patchy human rights record and has mistakenly bombed displaced civilians.
Billions of dollars were allegedly used for bribes in an unsuccessful bid to get ex-president Goodluck Jonathan re-elected.
Nigeria has suffered from falling oil prices and a lack of foreign exchange.
The Nigerian military has said it killed Shekau on at least three occasions.
Buhari led African leaders in expressing solidarity with the victims of the London attack.
Yemi Osinbajo took the reins as acting president during Buhari's absence for medical treatment in Britain.
The president said he would need to undergo "follow-ups" in the coming weeks.
Monday a presidential adviser had urged Nigerians to reconsider their trip.
The Nigerian government said several valid visa holders had been turned back.
A report alleged that the U.K. offered to rescue the girls, but Jonathan declined.
Kanu still faces five charges, including treasonable felony and terrorism.
Nigeria hopes world powers will supplement the government's efforts to fix the destruction Boko Haram left behind.
Seven suicide bombers blew themselves up near a refugee camp on Friday.
Buhari has been in London for almost a month, sparking rumors about his health.
The protests went ahead despite a star singer dropping out.
Muhammadu Buhari, 74, left Nigeria on January 19 to undergo "routine check-ups" in the U.K.
The singer has called for a peaceful march against the country's economic crisis.
James Ibori was released from a U.K. prison in December 2016 after his 2012 conviction.
If civilians continue to become collateral damage they will not support the government's battle against Islamist militants.
The president's decision has drawn criticism from some Nigerians.
President-elect Adama Barrow is due to take office on Thursday, ending Jammeh's 22-year reign.