Police said 73 students were abducted in yet another school attack in northwestern Nigeria, prompting authorities to close all primary and secondary schools in Zamfara state.

The new kidnappings occurred just days after three other groups of hostages were reportedly released after large ransom payments were reportedly made, raising hopes that other captives would be released soon as well.

Attackers descended upon the Government Day Secondary School in the remote village of Kaya around noon Wednesday, local resident Yusuf Mohammed told The Associated Press. The kidnappers then began shooting into the air before taking the students, he said.

Zamfara state police spokesman Mohammed Shehu said an operation was underway to rescue the students.

More than 1,000 students have been kidnapped from schools in northern Nigeria since December. While most pupils ultimately have been released, some have died or been killed in captivity and about 200 remained hostages before Wednesday's attack, according to UNICEF.

Government officials haven't commented on whether they played any role in the hostage releases announced Friday, but it appears parents from at least one of those schools did pay a large ransom.

The head teacher at one of the schools in Niger state told AP that many parents sold most of what they owned in an effort to raise funds totaling more than 30 million naira (about $72,900). The Salihu Tanko Islamiya School also sold off a piece of land where they had planned an expansion project, he added.

Those 90 pupils freed were the youngest hostages ever taken from a school in Nigeria, with children as young as 4 taken into the remote forests by gunmen and held for three months without their parents. One child, who hasn't been identified, died during the ordeal, authorities said last week.

It remains unclear whether the kidnappers of the three separate hostage groups last week are connected or if the simultaneous releases were merely coincidental. Each took place in a different state and they involved students of varying ages.

Authorities so far have blamed this year's spate of kidnappings on "bandits," or criminals operating out of remote, forested areas of northern Nigeria. Most of the gunmen are believed to be young men from the Fulani ethnic group who had traditionally worked as nomadic cattle herders before turning to the profitable crime of abducting children for ransom.

Some fear the gunmen in the northwest are linked in some way to the Islamic militants long active in the northeast, who drew international condemnation in 2014 when they abducted 276 schoolgirls in Chibok in 2014, prompting the #BringBackOurGirls campaign.

More than 100 of those girls are still missing, though two recently turned up years later, both of whom had had children with the militants they were forced to marry.