Beijing (CNN) Search crews on Friday raced to find a missing 9-year-old Chinese girl in caves and waters along the East China Sea, as the deepening mystery of her weeklong disappearance continued to grip the nation.

In live videos streamed by state-run media outlet Beijing News, crews on speedboats were seen to be deploying sonar equipment as they tried to detect signs of Zhang Zixin in waters near Xiangshan county in the eastern Chinese province of Zhejiang. Drones may be used as well, according to the search teams.

Zhang, a bespectacled second-grader with a long pony tail, was last seen in security camera footage from around 7:30 p.m. Sunday, when she appeared on a road not far from the current search area, along with a middle-aged couple who had been renting rooms at her house in rural Chun'an county, some 150 kilometers (93 miles) away.

When another surveillance camera in the area caught the two adults almost three hours later, the girl was nowhere to be seen. The man and woman, both hailing from the southern province of Guangdong, drowned themselves in a nearby lake later that night, local police said in a statement released Wednesday.

The girl's government services card was found in a pavilion along the Xiangshan coast on Wednesday night, according to the authorities, who announced a 10 kilometer (6.2 miles) radius search zone.

In an interview Wednesday with the mass-circulating Dushi Kuaibao newspaper, a Zhejiang-based state-run publication, the girl's father, Zhang Jun said the tenants had tricked his elderly farmer parents, who were Zixin's primary caretakers, into letting them take the girl away while he was working in a northern Chinese city hundreds of miles away.

Zhang Jun recalled how the male tenant -- who he identified as Liang Denghua -- and the woman -- who was only identified by her surname Xie in police statements -- claimed on July 3 to want to make Zixin a flower girl at a wedding in Shanghai. Despite his objections, Zhang said Liang and Xie led the girl away the next morning.

For three days, Zhang Jun said he was able to stay in touch with Liang via WeChat, China's most popular social media platform. Liang promised to bring the girl home by the end of July 6 and sent frequent updates, according to the father, who said everything seemed "normal" initially.

In footage Liang sent to Zhang Jun, who later shared with the Beijing News, Zixin can be seen visiting tourist sites with the two adults and watching videos on a phone on a train. The girl appeared relaxed, answering Liang's questions with a smile. Her voice in audio messages also sounded relaxed.

The problem: None of the locations was anywhere near Shanghai.

Zhang Jun said he became increasingly suspicious and finally took an overnight train home last Saturday. He said he spoke to Zixin for the last time midday Sunday. The father said his daughter sounded calm and told him she was in northern Xiangshan.

Chinese rescuers search for the 9-year-old missing girl Zhang Zixin on July 11 in Zhejiang province.

By Sunday night, things had started to look more ominous. Liang had refused the father's offer to drive to their location to pick Zixin up, but agreed to have him pay for a taxi to drive them back to Zixin's home, according to Zhang Jun, who added that Liang switched off his phone shortly afterwards.

Zhang Jun reported his daughter missing to the police on Monday morning, and posted Zixin and Liang's information online the next day. After police confirmed the death of the two adults, Zixin's fate has become a nationwide fixation.

State media outlets have been providing breathless coverage on the story, with journalists descending on the hometowns of Zhang, Liang and Xie to uncover details ranging from Zhang Jun's broken marriage, Liang's mental state to Xie's past debts.

As internet users scrutinize social media platforms, speculations and conspiracy theories also abound across Chinese cyberspace, with some commentators wondering aloud if Liang and Xie were members of a cult -- based on certain images and seemingly odd content posted by Liang.

Many online posts have also pointed to another coincidence: Zhang Jun and his long-estranged wife formally filed for divorce on Monday, when Zixin was already missing.

Zhang's now ex-wife, who only wanted to be identified by her surname Zeng, told the Beijing News on Thursday that she and Zhang Jun had been living different provinces and the couple had planned on the filing date in advance. Zeng said she hadn't seen her daughter for three years and wasn't aware of her missing until Wednesday, two days after the divorce.

CNN's repeated phone calls to Zhang Jun went unanswered Friday and has been unable to locate Zeng.

By midday Friday, however, hopes of finding Zixin -- alive or dead -- had faded as the search operation was called off amid rainy weather and choppy sea.