Environmentalists are offering a €30,000 ($32,950; £26,789) reward for information about the presumed death of a wolf in Belgium.
Last year, the wolf - named Naya - became the first to be sighted in Belgium for a century.
But local nature agencies said neither she nor her cubs had been spotted since May, and suggested batteries on her tracking collar had worn down.
Several nature groups have blamed hunters for their disappearance.
The Nature and Forest Research Institute (INBO) declined to say how the wolf and her cubs had been killed as there was no trace of them. However, a spokesman said there was a dense network of cameras in that area of Flanders, so it was definite that Naya was dead.
Three private environmental groups - Bird Protection Flanders, Animal Rights and the Nature Aid Centre - initially offered €20,000 for information.
Bird Protection Flanders announced on Thursday that an unnamed Belgian businessman gave them €10,000 more towards the fund.
"Everything shows that the public no longer accepts that some people give themselves the right to decide which animals are allowed to live," the group wrote in a statement.
Speaking to local media, a Flemish hunters' association called the accusations "scandalous," and said it may file a defamation suit against two Belgian conservation groups.
Naya, originally born in east Germany, was first spotted in the northeast Belgium in January 2018. She was later joined by a male wolf, named August, who became her companion.
The INBO spokesman told the BBC that Naya last appeared on their camera network on 10 May. At the time, she was pregnant and close to giving birth.
August was also filmed bringing prey to their den for the cubs. But the institute said he stopped doing this two weeks later and was behaving like a solitary wolf again - another sign that Naya and her cubs were dead.
August is one of three wild wolves left in Belgium. The other two are in the French-speaking Wallonia region.
Wolves were hunted out of many European countries over a century ago but have gradually been migrating back across the continental mainland.
Their return has proved controversial, with farmers complaining about attacks on livestock.