The search has intensified for a 9-year-old girl from Zhejiang province whose mysterious disappearance almost a week ago has grabbed hearts and headlines across the country.

Zhang Zixin lived with her paternal grandparents in Chun'an, the rural part of the province's capital, Hangzhou. She was taken from her home on July 4 by a couple who had rented a room in her grandparents' house for less than two weeks.

The couple told the grandfather that they wanted to borrow the child as "a flower girl for a friend's wedding in Shanghai" and promised to bring her back afterwards.

The family lost contact with the girl on the afternoon of July 7.

On the morning of the next day, the bodies of the couple, both aged in their 40s and from Guangdong province, were found in a lake a one-hour drive from the spot the last image of the girl was recorded at. Local police said it's likely that the couple committed suicide by drowning.

The girl was last seen with the two suspects in surveillance camera footage taken on Sunday night near Songlanshan Beach.

The couple were spotted by a camera three hours later in a taxi without the girl. The taxi driver recalled hearing no conversation between the couple over the hour-long ride, according to Beijing News.

Land searches since Monday around the area Zhang Zixin was last seen at have found no trace of the girl and efforts have shifted mainly to the sea.

A statement issued on Thursday evening by authorities in the province's Xiangshan county, said the search force had increased from 300 to 500 people, including police, fishermen and local residents.

The search area has expanded from 3.7 kilometers to 37 km with the help of drones, rescue dogs and speedboats.

The search will continue over the weekend, authorities said.

Zhang Zixin's grandmother said that the couple had been living in a motel next to her fruit store for two weeks before asking to rent a room. The offer was made in late June, soon after the couple saw the girl. The couple canceled their flights to leave after the grandparents agreed to the rental of 500 yuan ($73) per month, the grandmother said.

The girl had been living with her grandparents for years after her parents separated and each left to work in bigger cities.

The girl's father was in close contact with the couple as they took his daughter on the road, receiving videos and photos every day till Sunday afternoon. He called the police on Monday morning.

After the case sparked national attention, the mother fell under suspicion when she returned to Chun'an to sign divorce papers on Monday. The mother, who gave birth to the girl at the age of 17 and hasn't seen her daughter in years, said she knew nothing of the girl's disappearance till Wednesday, when a relative told her about it.

One of the suspects, surnamed Liang, had a son and daughter back in his hometown, but rarely visited or contacted them. The brother of the woman, surnamed Xie, said the couple had been living together for over a decade but never married.

Based on Liang's social media postings, the pair had traveled to 21 cities since March.