Spanish authorities are building their case against Mitchell Ong, 43, the Singaporean man suspected to have been involved in the killing of fellow Singaporean Audrey Fang, 39, in Spain.

Unique footwear

One piece of crucial evidence linking Ong to Fang's body is a S$1,362 pair of limited edition, European size 48.5 Nike sneakers.

It is an exclusive pair of sneakers that retails on luxury clothing website Farfetch and its size is consistent with someone of Ong's 1.88m height.

The soles of the shoes apparently matched footprints seen at the location in Abanilla where Fang's body was found.

Spanish media outlet La Verdad de Murcia reported that investigations have focused on a €935 (S$1,362) pair of shoes recovered from Ong's hotel room in Alicante.

He was arrested there.

He was reportedly due to meet his Ukrainian girlfriend at the same hotel the following day.

Ong got married in 2012, has a daughter, lives in a Bukit Timah condominium and owned several businesses in Singapore, it was previously reported.

Soil sample sent for analysis

A sample of the soil lodged in the soles of Ong's shoes were sent for laboratory analysis to find out if it matches the soil found on Fang's body, as well as with the sandstone found in the Abanilla area, reported La Verdad de Murcia.

The Spanish news outlet reported on these details derived from court documents.

Ong seen wearing different clothing

On Apr. 9 when Fang was last seen, surveillance footage at the hotel where Ong was staying showed him leaving in a hoodie and black pants, at around 5:45pm.

A judge overseeing this case noted that Ong was wearing different clothing when he returned a few hours later.

He returned to the hotel at around 2:06am on Apr. 10, wearing jeans and a blue sweater.

Ong and Fang had known each other for many years, it was previously reported.

Ong and Fang were together based on phone records

CNA reported that Ong and Fang's mobile phones were together in Abanilla on the day she died, based on what Spanish police can prove from data provided by mobile phone operators, citing Spanish newspaper Levante El Mercantil Valenciano.

Fang was not sexually assaulted, it was reported.

Flight risk

Ong is considered to be a flight risk, as the Singaporean man has no roots in Spain and can leave the country any time as he has "sufficient economic capacity”.

The judge pointed out that cash in the form of €3,760 (S$5,474) and S$1,000 was found in the hotel room and seized.

Levante El Mercantil Valenciano also reported that Fang had reportedly transferred money to an unidentified third person, so there could have been an "economic motive" for the killing.

Sources also told La Verdad that Fang had participated in a type of investment fund.

On Apr. 9, Fang left her belongings in her hotel and was never heard from again.

She was travelling alone in Spain at that time.

A preliminary autopsy report found that Fang's cause of death were knife wounds and head trauma, reported La Verdad.

Top photo via Mitchell Ong Instagram, Fang family