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.”