The biggest wave of Turkish emigration occurred in 1989, when 360,000 left Bulgaria as a result of the communist Todor Zhivkov regime's assimilation campaign, but around 150,000 returned between 1989 and 1990. That program, which began in 1984, forced all Turks and other Muslims in Bulgaria to adopt Christian names and renounce all Muslim customs. The motivation of the 1984 assimilation campaign was unclear; however, many experts believed that the disproportion between the birth rates of the Turks and the Bulgarians was a major factor. The official government claim was that the Turks in Bulgaria were really Bulgarians who were Turkified, and that they voluntarily chose to change their Turkish/Muslim names to Bulgarian/Slavic ones. During this period the Bulgarian authorities denied all reports of ethnic repression and that ethnic Turks existed in the country. During the name-changing phase of the campaign, Turkish towns and villages were surrounded by army units. Citizens were issued new identity cards with Bulgarian names. Failure to present a new card meant forfeiture of salary, pension payments, and bank withdrawals. Birth or marriage certificates would be issued only in Bulgarian names. Traditional Turkish costumes were banned; homes were searched and all signs of Turkish identity removed. Mosques were closed or demolished. Turkish names on gravestones were replaced with Bulgarian names. According to estimates reported by the Federal Research Division of the US Library of Congress, 500 to 1,500 people were killed when they resisted assimilation measures, and thousands of others were sent to labor camps or were forcibly resettled.