Most of the threatening mobile phone text messages and website images that spread panic among migrants from cities in the south and west of India last week originated in Pakistan, India's interior ministry said on Saturday.
Thousands of students and workers from India's northeast fled Mumbai, Bangalore and other cities, fearing retaliation for recent violence against Muslims in Assam, one of the states in their far-flung corner of the country.
The false rumours of Muslim revenge attacks were fuelled by threats posted on social media websites and spread through text messages. Migrants were also alarmed by misleading images that purported to show victims of mass killings.
"After checking and verifying we are saying, with responsibility, that the bulk of SMSes (text messages) spreading rumours about the northeast region have come from Pakistan," an interior ministry spokesman said, declining to be named for security reasons.
Clashes between indigenous people in Assam and Muslim settlers from neighbouring Bangladesh have killed 75 people and displaced more than 400,000.
More than 30,000 people form the northeast may have fled cities in the south and west, many of them on special extra trains that had been laid on, media reports said on Saturday.
Ringed by China, Myanmar, Bangladesh and Bhutan, India's northeast is home to more than 200 ethnic and tribal groups. Their facial features make them stand out in other states and many migrants from the region are considered Chinese or Nepali.
The ministry spokesman did not say who might have sent the text messages and posted the website images.
Arch-rivals India and Pakistan, who have fought three wars since their independence 65 years ago, regularly accuse each other of provocative acts.
There was no immediate reaction from Islamabad.