Three more cases have been confirmed at the stables of trainer Donald McCain as British horse racing is shut down by an equine flu crisis.
One of the horses, Raise A Spark, ran at Ayr races on Wednesday and had shown no symptoms of the highly contagious virus beforehand.
That takes the total number of cases to six at McCain's Cheshire stables.
Three separate suspicious cases at the base of trainer Rebecca Menzies are also being investigated.
As part of a six-day racing shutdown, all meetings have been cancelled in Britain until Wednesday at the earliest by the British Horseracing Authority.
A total of 174 racing stables were in lockdown on Friday afternoon as testing continued.
Menzies said test swabs had been sent for analysis to the Animal Health Trust in Newmarket "as a precaution following the sickness of three horses stabled in an isolation yard on our premises".
She added: "At this stage we have not received the results, our horses in training all appear healthy and thriving and of course I fully support the actions of the BHA and I look forward to a swift conclusion for our industry so we can get back to providing enjoyment for so many."
No link has been established at this stage between the two stables.
Menzies had runners at Newcastle on Tuesday and Wolverhampton on Wednesday.
As a result, the BHA has ordered dozens of others who saddled runners at those fixtures into lockdown mode.
Equine influenza is not markedly different from human flu - infected horses suffer with coughing and runny noses and generally feel rundown.
"The BHA is working closely with the Animal Health Trust in order to manage the logistical challenge of providing sufficient swabs and handling the volume of tests being sent through the facility, considering that testing of a total of 174 yards is now taking place," said the authority in a statement.
"It will not be possible to test every horse from every yard before the end of the weekend, but we will work with trainers to identify any priority or risk horses and ensure that they are tested. This will all form part of the picture that is built in order to assist the decisions that will be made on Monday."
It is expected that a clearer picture will emerge on Monday of when racing might resume, and any potential impact on next month's showpiece Cheltenham Festival.
Nine sites have suffered confirmed outbreaks of equine flu since the start of 2019, although only some involve racehorses.
These include a report involving eight vaccinated two-year-old thoroughbreds in Suffolk on Monday, and a vaccinated horse in Leicestershire on 30 January.
Other reports involving unvaccinated horses have come from Essex, Cheshire, Derbyshire, Lincolnshire, Yorkshire and another case in Suffolk centred on six non-thoroughbred horses.
While it is not thought to be life-threatening, equine influenza limits the ability to compete and it is very contagious.
Cornelius Lysaght, BBC horse racing correspondent
The stakes have risen markedly, I think. More positives from the McCain team is perhaps no surprise - everyone keeps saying how easily the virus spreads - but that one of them was a horse that travelled away to the races this week, coming into contact with other horses, is clearly a significant development.
Trainers who were previously quietly optimistic they'd get the all-clear must now be more nervous.
Meanwhile, as a result of other "suspicious cases", unconnected to McCain, the number of stables in lockdown is over 170 - that's a big number; this problem is now escalating.
How has the situation unfolded?
The crisis was sparked on Wednesday night when three vaccinated horses tested positive at the Cheshire stables of Grand National-winning trainer Donald McCain - whose late father Ginger saddled the legendary Red Rum to a record three National wins in the 1970s.
Because other horses from McCain's yard had raced at Ayr, Ludlow and Wolverhampton this week, the BHA ordered all four meetings on Thursday to be cancelled.
This was later extended to a total shutdown of racing until Wednesday at the earliest, with more news expected on Monday when the results of tests are known. Racing is continuing as normal in the Republic of Ireland.
Newbury's 'Super Saturday' meeting - where last year's Cheltenham Gold Cup winner Native River was due to have a final race before his title defence - is among the casualties, although there are hopes some of the big races may be rescheduled for a later date.
The yards in lockdown include the Lambourn, Berkshire base of champion jumps trainer Nicky Henderson, who has saddled more winners at the Festival than anyone else.
"We did have two horses running at Ludlow on Wednesday where there was a horse that has come from Donald's yard, and it could have been in contact with an infected horse," Henderson, who trains champion hurdler Buveur D'Air and champion chaser Altior, told BBC Radio 5 live.
"It is very, very virulent so it does not matter how much pains you go through to keep these things at bay, it is very dangerous.
"We are all totally vaccinated, rigidly, it's compulsory. Unfortunately, I am told there is only 40% of the horse world - of the horses in Britain - that are actually vaccinated. There is no jurisdiction for horses outside of racing to say that you have to be vaccinated.
"Like in humans, there are many, many strains of it. So it is possible there is a new one that you don't know about."