Six members of boxing teams who attended an Olympic qualifying event in London in March are said to have contracted coronavirus.
The Turkish Boxing Federation said two fighters and a coach tested positive since returning from the event.
Marko Marovic of the Croatian Boxing Federation also claims a fighter and two of its coaches have the disease.
He told BBC Sport: "Our epidemiologist says they were most probably infected during [the] Tokyo 2020 qualifier."
Marovic, secretary general of the Croatian Boxing Federation, added: "Our first concern is the health of our boxing family. We are still praying that everything ends well and there will be no other positive tests."
He said both Croatian coaches and the fighter were "stable" and in quarantine.
Earlier, organisers of the Road to Tokyo competition at the Copper Box - including an International Olympic Committee (IOC) Boxing Task Force - were branded "irresponsible" by the Turkish Boxing Federation after three of its team tested positive.
The event was called off after three days because of the pandemic.
Turkish Boxing Federation president Eyup Gozgec told BBC Sport that organisers failed to take the outbreak seriously enough and "didn't care".
He added: "I just want to know, when the whole world was on high alert, why they hosted this event?
"We didn't see any hygienic standards there. There were no preventative measures."
The IOC said its Task Force was "not aware of any link between the competition and the infection".
It added that at the time of the competition there were no governmental restrictions in place, but expressed its sympathy for the affected athletes.
The local organising committee also said they had applied extra precautions and that no teams had reported symptoms at the event.
In a scathing letter sent to the board members of the European Boxing Confederation (EUBC), and seen by the BBC, Gozgec wrote: "Two of our athletes (and a coach) have tested positive for (Covid-19) after returning to Turkey from London. All of them are in treatment now and thankfully are in good condition.
"This is the disastrous result of the irresponsibility of the IOC Boxing Task Force."
Gozgec says he now intends to write to all European boxing associations asking if any of their teams at the event have since tested positive.
"The organisers were irresponsible, and I think they didn't realise the severity of the issue so they just didn't care," he said.
"They just didn't take this outbreak seriously and they didn't care about it. They did no tests for us. They just told us to go. They dropped us at the airport and that was it."
He added: "They knew they were going to have to cancel - why go with it? The health of our athletes and staff is our priority."
It is not clear whether the fighters and coach contracted the virus in London or once they returned to Turkey.
Organised by the IOC's Boxing Task Force (BTF) following the suspension of international federation the AIBA last year, the European qualifying event started on 14 March with full crowds at the Copper Box, the day after major sporting events including all professional football in England, Scotland, Wales and Northern Ireland had been suspended.
After the first two days of the event, the BTF decided that "due to the changing situation with coronavirus and concerns for public, athlete and volunteer welfare", the event would take place "behind closed doors".
However, following the third day of competition, the event was suspended.
In a statement, the IOC said: "The BTF wants to express its sympathy for the affected athletes and officials and wishes them a very speedy and full recovery.
"The BTF is not aware of any link between the competition and the infection.
"Many participants were in independently organised training camps... before the competition started... and have returned home a while ago so it is not possible to know the source of infection.
"At the time of the European qualifier in London there were many sports and other events going on in Great Britain because there were no governmental restrictions or advice on public events in place.
"Precautionary measures before, during and in the follow-up phase of the event were implemented and the event was suspended when the Covid-19 situation developed further.
"Safeguarding the well-being of the athletes, officials and all other participants has always been a top priority for the BTF."
A spokesperson for the event's Local Organising Committee said: "Extensive measures were put in place to protect the health and welfare of participants during the competition.
"Our medical team was in daily contact with Public Health England throughout the tournament and provided regular guidance to all of the teams which reflected the Government's advice.
"A range of extra measures were put in place to support teams which included the provision of hand sanitisers throughout the competition spaces and routine temperature tests which enabled the medical team to track and identify any underlying changes during competition.
"Teams were advised on the steps they should take if any members of their delegation showed symptoms of the virus.
"During the competition there were no recorded instances of teams contacting the medical team to report symptoms."
Gozgec's letter continued: "This virus has been around since December 2019. Therefore, it is inevitable to ask why the European Qualification event was not postponed before it even took place?
"The price of excluding AIBA and EUBC will be a hefty one for the IOC Task Force. Do they possibly think that their problems with individuals grant them the right to put athletes and their families in such jeopardy?
"Henceforward, I ask the IOC Task Force for why they hosted an event that they were going to cancel merely after three days?
"It is evident that nobody put forth a slight effort to organise this event in a hygienic way to protect athletes, coaches and the staff involved. They did not consider anyone's health, which led them to organise this horrible event."
Gozgec also criticised the IOC Task Force for "requesting a full up-front payment from all of us boxing federations in Europe for 15 days".
Following the suspension of the event after three days, he has requested information on whether they will be reimbursed.