Three further Hull FC players have tested positive for Covid-19 - taking the total number to nine.
Another member of staff has also tested positive, meaning 12 individuals in all have returned a positive test following Hull's Super League defeat by Salford last weekend.
Salford have not returned a positive test in two separate rounds of testing this week, with another two scheduled for next week.
Seven of the nine players were in action against the Red Devils, with both clubs having their scheduled Super League games for this weekend postponed.
It has also been confirmed that Hull's Challenge Cup sixth-round tie against Castleford next week will not take place.
Hull chairman Adam Pearson told BBC Radio Humberside that the club is investigating the possibility that the virus could have been brought to the club after one player attended hospital.
He said: “It’s very difficult to say too much publicly but it appears one of the players’ newborn baby has required hospital treatment.
"That player has gone to hospital and subsequently some members of his family could have been infected.
“He’s then come in on Saturday for some treatment with symptoms and been sent home.
"He did a rehab session or was about to start but was sent home and I think it’s probably gone from there to the physio and into the players on Saturday and Sunday morning.
“That’s not a scientific answer, that’s where I’m looking at it as the most likely cause.”
