Thai authorities have reportedly asked for help from Malaysia in tracking down the owner of a mobile phone used to detonate one of the explosives in the recent wave of bombings that targeted some of Thailand's best-known tourist destinations.

Bernama, Malaysia's state news agency, reported on Sunday that the owner of the mobile phone, recovered from one of the blast sites in the popular island of Phuket, was "said to be from Malaysia". 

The phone was only partly damaged in the explosion, leaving its serial number still visible to investigators, a security source, who spoke on condition of anonymity, told Bernama.

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According to the Bernama report, the serial number belonged to the Malaysian Communications and Multimedia Commission (MCMC), a regulatory body overseeing communications in Malaysia, including phone and online activities.  

Thai police have forwarded the phone fragment to Malaysian authorities. 

In a statement on Sunday, MCMC said it would "work closely with police" to help find the suspect.

A wave of explosions hit resort areas in Thailand on Thursday and Friday, killing at least four people and wounding 35 others, including tourists. 

Also on Sunday, Thai police said the attack had been orchestrated by a single person, without giving any further details. Authorities said they "know who to blame" but have yet to reveal a culprit or suspected motive. 

"Our investigation is progressing. We know who was behind it," Piyapan Pingmuang, the deputy national police spokesman, told AFP news agency, declining to give further details.

Found and defused

No one has yet claimed responsibility for the bombing spree, which hit seven southern provinces.

On Sunday, authorities said they had found and defused more unexploded devices in two bomb-hit areas - the upmarket beach town of Hua Hin and Phuket.

"It is likely they were intended to explode at the same time of the previous explosions," said Grisada Boonrach, a Thai interior ministry spokesman. 

Other unexploded devices were found on Saturday in Phang Nga province.

Thai police close in on tourist attacks' suspects

At least three men have been arrested or held for questioning following the attacks, police said.

A spokesman for Thailand's military government confirmed that multiple people had been questioned but stressed it was too early to identify them as suspects.

"It's just asking questions. They will not be treated as suspects unless the questioning procedure is done and any of them are found to have violated laws," said Colonel Winthai Suvaree.

Thailand has been battered by a decade of political unrest, driven by a bitter power struggle between the military-allied elite and forces loyal to the ousted democratically elected government.

Muslim rebels are also waging a long-running armed struggle in Thailand's southern tip. 

The bombings in top tourist destinations threaten a vital source of income for tropical Thailand.

The sector accounts for at least 10 percent of an economy the military government has struggled to revive.

Source: Al Jazeera and agencies