The plane entered Belarusian airspace in July and air dropped over Minsk and elsewhere cloth bears stuffed with protest slogans, RIA Novosti reported Monday.
Media called the toys "Teddy bear paratroopers."
Investigators found that a border guard patrol detected the plane when it crossed the Belarusian border from Lithuania and reported it to the border post. But the person on duty didn't report the information to the unit officer, violating procedure, RIA Novosti said.
Belarusian President Alexander Lukashenko fired State Border Committee Chairman Igor Rachkovsky and Air Force and Air Defense Commander Dmitry Pakhmelkin, and a number of military and security officials were reprimanded.
The Swedish ad agency said the Teddy bear air drop was meant to draw attention to human rights abuses in Belarus, the Los Angeles Times reported. Human rights groups have labeled Belarus as one of the most repressive countries in Europe.
Belarusian officials tried to summon the Swedish advertising team for questioning, but the Sweds rejected the summons, the Times said. Ad agency officials said they would meet with Lukashenko in Sweden but only if he released "all the political prisoners in Belarus."
Human Rights Watch said in its latest annual report that at least a dozen political prisoners are in Belarusian jails.