Save the Children has launched an urgent investigation into how a Scottish nurse contracted the Ebola virus in Sierra Leone, as health chiefs promise to review screening measures in the UK.

The charity’s humanitarian director, Michael von Bertele, said he had ordered an inquiry into whether Pauline Cafferkey caught the disease outside the treatment centre in Kerry Town where she had volunteered.

He told the BBC Radio 4 Today programme: “I would say that protection, if people adhere to the protocols, is of a very high standard but … nothing is risk-free, particularly when dealing with a disease like Ebola.”

He said the investigation had started already.

“It’s really important for us to try and understand whether it was a failure of training, of protection, of procedure, or indeed whether she contracted it in some incidental contact within the community, because our workers don’t just work inside the red zone, which is a very high-risk area, they do also have contact - although we are very, very careful in briefing people to avoid personal contact - outside of the treatment centre.”

Nurse Pauline Cafferkey. Photograph: PA

Cafferkey, from Blantyre, South Lanarkshire, contracted the virus while carrying out a five-week volunteer placement for Save the Children. She returned to the UK on Sunday night and told officials at Heathrow she felt unwell but was allowed to catch a British Airways flight to Glasgow.

The 39-year-old was diagnosed with Ebola the next morning and is receiving specialist treatment in a high-level isolation unit at the Royal Free hospital in London. A spokesman for the hospital declined to comment on her condition on Wednesday morning, but said an update would be issued in due course.

Dr Martin Deahl, a fellow NHS volunteer who returned to the UK with Cafferkey, told the Guardian that she almost certainly would have contracted the virus outside the treatment centre, which has strict protocols to protect staff and patients.

“I’m sure she did [contract it] outside the treatment centre,” he said. “I can’t imagine that anyone following the procedures and protocols in the treatment centre would have contracted it.”

Save the Children runs an 80-bed treatment centre in Kerry Town, which is believed to be where Cafferkey was based. A spokeswoman for the charity confirmed that it has volunteers working at the facility but could not say whether they were from the UK or whether more NHS health workers were due to fly out.

The Department for International Development said it expected more volunteers to be trained and deployed in the coming weeks. A DfID spokeswoman said a team of 25 travelled to Africa on 6 December and a dozen medics flew out on 21 December.

The chief medical officer, Dame Sally Davies, conceded that the case raised questions over Ebola screening precautions.

“We regularly keep under review what we are doing because this is a new process,” she told ITV’s Good Morning Britain.

“Clearly, queuing and things like that are unacceptable and we will review. But we will let people who are well travel because they will not infect the public.”

She said Cafferkey had been well. “She had no symptoms. Her temperature was within the acceptable range. She would not be transmitting the virus, therefore she was cleared as fit to fly.”

A Department of Health spokesman said: “We have been clear – this person was tested as part of the screening process at Heathrow and, as with all health workers, she was advised to contact PHE [Public Health England] if she had any concerns. She did this while still at Heathrow and went through a further six temperature checks.

“Her temperature was in acceptable ranges and she was cleared for onward travel, with the advice that if she did start to feel unwell, she should contact health authorities.

“After this person got home, she alerted health authorities that she was experiencing symptoms and was admitted to hospital for tests. Naturally, we will be reviewing what happened and the screening protocols, and if anything needs to be changed it will be.”