LONDON — One of the suspects detained in Britain over last week’s London subway bombing is an 18-year-old refugee from Iraq and the other is a 21-year-old thought to be from Syria, British media said.

Both men were fostered by a British couple.

The disclosure came Monday as police stepped up their investigation into the attack on a packed London Underground train on Friday during rush hour that injured 30 people.

Commuters returned to Parsons Green station Monday for the first morning rush hour since the bombing. Police asked travelers to be vigilant and to expect more armed police on the transit network.

According to the BBC, the 18-year-old moved to Britain at 15 after his parents died. The 21-year-old, named by the British broadcaster as Yahyah Farroukh, is believed to have been in the country for at least four years. Police have not commented.

The pair were detained over the weekend and investigators are probing whether they knew each other and if others were involved in the plot. They are also trying to confirm any links the men may have to the Islamic State group, which claimed responsibility.

Mark Rowley of the Metropolitan Police said investigators have a "greater understanding" of how the bomb, smuggled onto the train in a plastic shopping bag, was assembled, but still have "much more to do."

Both suspects, the BBC reported, are thought to have spent time being fostered by Penelope and Ronald Jones in Sunbury-on-Thames, an area south of London.

The couple, 71 and 88 respectively, were previously recognized by the queen for their services to children and families, including refugees.

The British government last year pledged to take in 20,000 Syrian refugees by 2020. According to the most recent figured cited in a British parliamentary report, 5,453 Syrians were resettled to Britain in the year ending March 2017.