Bangkok — Thailand raised a reward Friday for tips leading to the arrest of the main suspect in Bangkok's deadly bombing and turned to the United States for help in tracking down those behind the attack that left 20 people dead.

Prime Minister Prayuth Chan-ocha said he had received offers of assistance from the U.S. Embassy in Bangkok and had assigned his deputy "to cooperate on borrowing equipment that includes facial-recognition technology."

Prayuth, however, ruled out working with U.S. investigators, insisting Thais can do the job.

"It won't be necessary to cooperate on the investigation with U.S. officers," Prayuth told reporters. "We need to help ourselves."

Four days after the explosion at the revered Erawan Shrine, at one of the capital's busiest intersections, there were few solid leads into the perpetrators of the attack that also left more than 120 injured.

Police were still searching Friday for the prime suspect seen in a security video dropping off a backpack near a bench at the site about 15 minutes before the blast, a day after clearing two other men seen in the video who were initially believed to be suspects.

National police chief Somyot Poompanmoung told reporters that police were looking for a woman wearing a black shirt who appeared in the footage, seated near the suspect. He noted she was not considered a suspect but could have valuable witness testimony, if police could find her.

"We don't even know who she is," Somyot said. When asked if any persons of interest would be called in Friday, he said, "No."

After being criticized for sending confusing messages, authorities appeared more guarded in their statements. Military spokesman Col. Winthai Suvaree said on television that the police were making "much progress" but that he could not disclose any details.

Police have released a sketch of the suspect — depicting him with eyeglasses and bushy, black hair — and offered a reward that on Friday was raised to 3 million baht ($85,000), Somyot said. On a police arrest warrant he is described as a "foreign man," although a military spokesman said Thursday that a connection to international terrorism seemed unlikely.

Somyot had signaled the need for facial recognition technology to help speed the investigation.

"There are automatic machines that can detect 100 people within 5-6 seconds," Somyot said Thursday. "Making people sit and do the job would take all day."

The U.S. Embassy confirmed that it had offered to help Thai authorities but declined to give specifics, citing the ongoing investigation.

"The Embassy in Bangkok has informed Thai authorities that we stand ready to assist with the investigation as needed," said embassy spokeswoman Melissa Sweeney. "We will continue to consult closely with local authorities regarding the attack and will provide assistance as appropriate."

At the site of the blast, several ceremonies were held Friday to mourn the victims and to show that the bustling capital was respectfully, if cautiously, moving on.

Doves were released into the sky Friday afternoon at the shrine, after a morning multi-religious prayer ceremony attended by Buddhist, Christian, Hindu, Sikh and Muslim priests and representatives. Government officials and diplomats laid floral bouquets at the shrine, an open-air Hindu temple popular with Chinese tourists.

Office worker Pratuang Limkul was among many Bangkok residents who also came to pay respects.

"I came to send the spirits of those in this place to rest in peace," she said, after kneeling in prayer.

Many of the victims from Monday's blast were foreigners. Among the 20 people killed, six have been identified as Thai and four as Malaysians, four from mainland Chinese, two from Hong Kong including a British citizen, one Indonesian and one Singaporean. Two victims remain unidentified.

No one has claimed responsibility for the blast, sparking a variety of theories into who might be behind it. One is that the blast was a revenge attack related to Thailand's recent deportation to China of more than 100 Uighur Muslims, or that it could have been carried out by Islamist groups expanding their reach in Southeast Asia.

Other speculation points closer to home. Muslim separatists have been waging a low-level but deadly insurgency in southern Thailand since 2004, leaving more than 5,000 people dead, but virtually all their attacks have been confined to the southernmost provinces.

There has been little violence aimed at Thailand's coup. Political violence boiled over during 2010 protests, when the "Red Shirt" movement that supported the ousted elected government clashed with the military, leaving about 90 people dead.

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Associated Press journalists Grant Peck, Tassanee Vejpongsa and Penny Yi Wang contributed to this report.