They will send experts to conduct tests at the Bahia Principe hotels where Schaup-Werner, Holmes and Day died.
The three other tourists died several weeks after Wallace.
Rojo, who is expected to leave Old Trafford this summer, was unaware of the shock deaths having been on a two-day fishing trip.
The Argentinian is understood to have continued his holiday elsewhere on the island.
"He was fine", Arnold said.
Just as Tommy Tickenoff was gaining a wife in the island country, his mother was losing her husband of 10 years. Felicia Nieves told WTXF her sister had a drink from the minibar inside her room, went to bed and never woke up. Immediately, "he started feeling very sick, he had blood in his urine and stool right afterward", his niece tells Fox News.
Daily Star Online has approached Rojo's representatives for comment. The man chose to go to the hospital on April 13 and died the next day.
Meanwhile, a DE woman named Tammy Lawrence-Daley said she was attacked in the Dominican Republic at another all-inclusive resort while she was vacationing with her husband and some friends.
"We were either going to lose all our money or we were going to get to go somewhere else".
The FBI is partnering with Dominican Republic authorities to launch an investigation into the four mysterious American deaths at Dominican resorts over the last two months.
Arnold said her uncle, an avid traveler, had been in relatively good health and just the month before had been skiing in Lake Tahoe. Among them was Miranda Schaup-Werner, 41, of Allentown, who was celebrating her ninth wedding anniversary at the Luxury Bahia Principe Bouganville.
On Friday, a Colorado couple told CNN they are suing the resort the Grand Bahia Principe La Romana resort, where Day and Holmes stayed, after falling ill last June. Her obituary simply states she died suddenly on June 23, 2018 while in the Dominican Republic.
We reiterate that we totally disagree with the dissemination of false information published publicly that endangers the image and reputation of the company and the integrity and rights of our employees and their families, reserving, when necessary, the right to undertake the adequate legal action.
Awilda initially thought it was an accident, but she now thinks she may have been spiked, following the deaths of three people at the resort last month. "I no longer feel like my husband died of natural causes". My mouth was on fire.
The resort apologised and offered her and her boyfriend a meal and a massage, according to reports.