On Saturday night, Tracy Paniagua boarded a bus with a group of girlfriends for what was billed as a lingerie party.

As the bus rolled through the Boston area, Paniagua, 26, danced to loud music while she videotaped the celebration and shared clips on Snapchat, her relatives said. Just before 2 a.m. Sunday, she posted a video from Revere Beach, according to her boyfriend, Marquis Brimage.

It would be her last.

“She was having fun. She was enjoying herself. Everybody was,’’ her cousin, Algecira Garcia, said Tuesday. “You don’t know what’s coming.’’

Around 2:25 a.m., Paniagua was killed after falling out of a bus window onto McClellan Highway in East Boston, authorities say. Boston police Lieutenant Detective Michael P. McCarthy said police are trying to determine whether Paniagua was then struck by the bus or an oncoming vehicle, and asked anyone who might have witnessed the crash to come forward.

The company that owns the bus, Entourage Livery, said two passengers fell out of an emergency window. The second passenger was injured.

Paniagua’s sister, Evelyn, said a detective told her the women fell out of the bus when a window they were leaning against gave way. The investigator said Paniagua was then struck by another vehicle, another relative said.

“We feel that there’s more to the story. The window is not going to just collapse, especially out of a bus, just because,’’ Evelyn Paniagua said Tuesday. “For me, my sister’s dead. Whatever we do, whatever we say is not going to bring her back. But we strongly believe that it was negligent on the company and the bus.’’

Entourage Livery, which is based in North Attleborough, sharply denied the allegations.

“We definitely don’t believe we were negligent,’’ said sales manager Tom Estrella.

On Monday, the company’s general manager said it was not clear how the emergency window was opened, saying, “It takes a good amount of pressure to pull the handle and push the window out.’’

Estrella said, “We know when the bus left the lot, the windows were secured.’’ The driver pulled over as soon as he learned what happened, he said, and has not been cited by authorities.

“It’s just an unfortunate tragedy,’’ Estrella said.

No charges have been filed, according to a spokesman for the Suffolk district attorney’s office.

The state Department of Public Utilities said the limousine company had passed its inspections since April 2009. The company has not had any crashes reported to the Federal Motor Carrier Safety Administration.

But since 2011, the federal agency has fined the company $37,500 for violating rules, including regulations about substance abuse and alcohol testing for drivers and hiring licensed drivers, records show.

The agency has allowed the company to operate with a “conditional’’ safety rating, meaning it could be prohibited from taking passengers across state lines if more violations were found.

Estrella said Entourage Livery has addressed all the concerns raised by federal authorities.

“We’ve done everything we’ve ever been asked and went above and beyond to make sure we were the safest,’’ he said.

Paniagua grew up in Dorchester and worked as a security guard at a women’s shelter in Mattapan, her family said. Her sister, Evelyn, said her family has not heard from anyone else who was on the bus.

She said her first inkling that something was wrong was a hunch. She read about the incident and became worried that Tracy was involved.

“I tried to go back to sleep, and I couldn’t,’’ Paniagua said. “When I felt that I was going to fall asleep, I saw her body in my dreams, in my mind . . . laying on the metal table.’’

The image made Paniagua reach for her phone.

“You got home?’’ she texted her sister a little after 7 a.m.

“[It’s] all I need to know,’’ she wrote a short time later.

At 7:36 a.m., she sent a third text: “Txt me ASAP. WANNA KNOW YOU OK.’’

Her sister never responded.

Paniagua’s niece, Sandralee Flores, 19, said she is comforted knowing that her aunt was “having fun all the way to the end.’’

“She was smiling,’’ she said.

Laura Crimaldi can be reached at laura.crimaldi@globe.com. Follow her on Twitter @lauracrimaldi.