Among those attending a vigil for the victims of America's latest gun massacre was one rather unusual guest.

Golden retriever Jacob, aged four, a specially trained “comfort dog”, could be seen lying in the shade, surrounded by a group of children and wearing a harness that simply said "please pet me".

Jacob is one of 19 dogs currently in Parkland supporting friends, family and loved ones as they grieve those lost when Nikolas Cruz opened fire at Marjory Stoneman Douglas High School shooting.

The dogs are part of a group of 130 golden retrievers, K9 Comfort Dogs, run by Lutheran Church Charities, which specially trains dogs to act as therapy animals in disaster situations.

“When someone pets one of our dogs it lowers their blood pressure and makes them relax.” Tim Hetzner, chief executive of the Illinois-based charity, told The Independent.

“This helps in the healing process of the individual and the community.”

The charity believes dogs help people feel calmer, and it's not alone.

A study last year by the University of Florida on pet-owning families found that dogs make children feel more supported socially and less stressed.

But what differentiates a specially trained comfort dog from a regular golden retriever?

According to Mr Hetzner, the comfort dogs all undergo over 2,000 hours of socialisation training which usually occurs over the span of a year and a half, beginning between eight and 10 weeks of age.

They are trained not to jump, bark, or display aggressive behaviour.

When they are not supporting people through disaster response, the retrievers typically work in libraries, schools, churches, with police officers and fire fighters, and at community events.

But it’s not all work – a day in the life of a comfort dog is mostly filled with “fun and rest”, Mr Hetzner says.

Florida shooting – in pictures Show all 10 1 /10 Florida shooting – in pictures Florida shooting – in pictures Police arrest a suspect in connection with the shooting at the Marjory Stoneman Douglas High School in Parkland, Florida Reuters Florida shooting – in pictures Parents wait for news after reports of a shooting at Marjory Stoneman Douglas High School in Parkland, Florida AP Florida shooting – in pictures Anxious family members wait for news of students AP Photo/Wilfredo Lee Florida shooting – in pictures Students being evacuated from the school AP Florida shooting – in pictures Students being evacuated from the school Getty Florida shooting – in pictures People gather waiting for word from students AP Florida shooting – in pictures Parents waiting for news on their children AP Florida shooting – in pictures People gather at a hotel where students were taken after the shooting Getty Florida shooting – in pictures Florida Governor Rick Scott speaks to the media as he visits Marjory Stoneman Douglas High School following the shooting AFP/Getty Florida shooting – in pictures Dr. Igor Nichiporenko, Medical Director Trauma, left, and Dr. Evan Boyer, Medical Director, Emergency Services, speak about treating victims and the suspect at a press conference outside Broward Health North hospital AP

Like many of the dogs, this is not Jacob’s first trip – he visited Orlando after the Pulse night club attack and Las Vegas after the shooting by Stephen Paddock.

So what’s next in store for Jacob?

“Jacob will serve in Florida to provide comfort and love,” Mr Hetzner said. “Following this deployment, he will enjoy a long period of rest and fun before returning to work.”