On May 31, 2011, Belinda ­Nguyen called 911 after finding her 12-year-old sister dead in the basement of their family’s Gaithersburg home.

“There’s blood everywhere — please help!” the teenager told the emergency operator. With her family members’ anguished screams in the background, she explained that she had seen “man footprints” on the floor and blood on the door.

That 911 tape was played Tuesday for jurors during the first day of David R. Hang’s first-degree murder trial in Montgomery County Circuit Court. Hang, 43, is accused of killing 12-year-old Jessica Nguyen after he had difficulty getting out of a sham marriage he had entered to help her mother, Khen Kim Vu, gain U.S. citizenship.

Prosecutors said in court that Jessica was stabbed more than 40 times. A weapon has not been found. But they said that a sheath found near the girl’s body and made to hold a small sword had Hang’s DNA on it.

“It was an angry personal murder, not a random act,” Assistant State’s Attorney Stephen H. Chaikin said in his opening statement.

Public Defender Brian Shefferman told jurors that Hang is “not a violent person in any way, shape or form.” He said that Hang, who had once lived in the house with Vu, harbored no bad feelings toward the family and maintained a close relationship to Vu’s children, even taking them to the mall and out to dinner.

“This so-called motive to kill a child is based off assumption, speculation and guesswork,” Shefferman said.

Hang and Vu were married in June 2006, authorities said. As part of the deal, they said, he agreed to see Vu through the application process for a green card. Once that was completed, the family said they would pay him $25,000 and then the cost of a divorce. They also helped him get a job as a Ride On bus driver.

By 2007, Hang and Vu were still married, but he had moved out of the family’s house. By 2010, Hang had a new girlfriend — whom he would later marry in Colorado — with health problems, authorities said.

Hang wanted to get coverage for her on his county insurance but couldn’t until he divorced Vu, Chaikin said.

Hang filed for divorce and Vu initially contested it, Chaikin said, creating financial and personal stress for Hang.

“Any one of these issues could overwhelm somebody,” he said. “The core of the defendant’s problems were with Jessica’s family.”

Eventually, Vu settled and the two were divorced, but Chaikin said that Hang had wanted it to happen much sooner.

Prosecutors said that experts concluded that the bloody footprints Jessica’s sister saw were from the same kind of boots Ride On bus drivers are issued. Hang’s boots have not been recovered.

Chaikin said that Hang had left work early that day and didn’t return.

He told jurors that cellphone tower records placed Hang near the family’s home about the time Jessica was killed. And after her death, he said, Hang’s right hand was swollen.

The trial is scheduled to continue Wednesday.