Determined Nebraska Sisters Seek Answers in the Mysterious Deaths of Their Parents Aboard Luxury Amazon Cruise

Kelly Hammer Lankford's heart sank as soon as the caller said he was with the United States Embassy in Peru, where her parents were vacationing on an Amazon River cruise.

She knew that the news couldn't be good. But nothing prepared her to hear that her mom and dad, Larry and Christy Hammer of Gretna, Nebraska, had died in their beds on April 10 after a fire broke out in their cabin aboard the luxury La Estrella Amazonica on the first night of a 10-day, 600-mile dream cruise.

Even more shocking was what she and her sister, Jill Hammer Malott, learned after hiring an attorney and private investigators in Peru to look into their parents' deaths, when they felt that they hadn't received enough answers from Peruvian authorities and the cruise operators, International Expeditions, based in Helena, Alabama.

Kelly and Jill's investigators concluded that the fire was caused by a faulty electrical power strip in their parents' cabin and that fire and smoke alarms weren't working. And video footage showed that it took crew members 22 minutes to pull their father from the second-story cabin, and another six minutes to rescue their mother, after somebody noticed smoke coming from a light fixture.

"In between, they opened the door to my parents' room and slammed it shut several times instead of rushing inside to get them out," Jill tells PEOPLE. "Our mom still had a heartbeat and could have been saved if they hadn't waited. There was failure aboard the boat on multiple levels."

Four months after the tragedy, she and Kelly are still trying to get conclusive answers from the cruise company and Peruvian authorities about everything that went wrong aboard the boat that night. They had Nebraska's congressional delegation write a letter to the Peruvian ambassador to the U.S., Luis Miguel Castilla, imploring him to make sure the investigation was timely and thorough.

The sisters are speaking out, they tell PEOPLE, to warn others about cruise ship safety at a time when interest in ecotourism trips is at an all-time high.

"It's horrifying to me that they had this boat back on the water less than 48 hours after the fire that killed our parents," Jill, 46, of San Francisco, tells PEOPLE. "We don't have all of the answers and yet they continue to load every week and put unsuspecting Americans back on the boat. We'd like to see documents showing that the boat is safe. We don't want anybody else to go through what we've been through."

Van Perry, president of International Expeditions, insists that the La Estrella Amazonica is safe and says the boat now has enhanced fire-fighting equipment and that the crew has been provided with refresher fire training.

"We'll continue to give this incident high priority," he tells PEOPLE in a statement. "We extend our sympathy to the family. We cannot attempt to understand the depth of their loss, but continue to provide our offers of support to them."

A 31-passenger boat, the La Estrella Amazonica was marketed to Americans as the largest, most modern vessel on the Amazon, says Kelly, which is one reason why her parents, who were avid world travelers, spent $10,000 to take the luxury cruise.

Larry, 74, a retired University of Nebraska-Lincoln administrator, and Christy, 72, a former schoolteacher and analyst for Gallup, had visited every continent except Antarctica, which they hoped to cross off their bucket list after their cruise on the Amazon.

"They were adventurous and generous people who had figured out what they truly loved in life – family and travel," says Jill. "They were great role models to Kelly and I when we were growing up, and to their four grandkids. 'Be thankful for what you have and live for what you love.' That was their philosophy."

The Hammers were initially scheduled to go on the maiden voyage of International Expeditions' "star of the Amazon" in 2013, but when the trip was delayed, they rebooked for this past spring, Kelly tells PEOPLE, departing on April 10 with 19 other passengers and 15 crew members.

Only eight hours into the cruise, at about 2:30 a.m., a fire broke out in their cabin, with no alarms sounding, while they slept. Larry was dead when he was removed from the cabin, while Christy died on the way to a hospital. Smoke inhalation killed them both, says Kelly, and her dad also suffered burns on his body since the surge protector that caught fire was located beneath his bed.

To make the sad outcome even worse, says Jill, her parents wedding rings have gone missing "and were likely stolen off their dead bodies," she tells PEOPLE.

Although Peruvian officials are still investigating, she and her sister say their attempts to talk to other passengers about the fire and any recollections of their parents' last moments at dinner that night have been rebuffed by the cruise company.

"The last time I talked to my parents was the day before they left when we called to say 'Bon voyage,' " Jill tells PEOPLE. "They were excited for the trip and promised my kids they'd take plenty of pictures of the animals they saw in the Amazon. Of course, they didn't get to take any pictures. We don't know about their last hours, but we're going to keep digging until we find some answers. No matter how small those moments may seem, they're huge to us."