ANKARA (Reuters) - Turkish security forces have detained 18 people attempting to illegally cross to Turkey from its Syrian border, including a Chechen man suspected of planning an attack, the military said on Tuesday.

The security forces found 1.5 kg (3.3 pounds) of explosives and two grenades in the Chechen man’s bag, a military statement said. The group was made up of nine other men, four women and four children.

Russia has been on alert against Russian-speaking rebels returning from Syria, where they have fought alongside Islamic State, and is wary of any attempts to resume attacks that dogged the country several years ago.

On Monday, a blast in a St. Petersburg train carriage killed 11 people and wounded 45. It was carried out by a suspected suicide bomber with ties to radical Islamists, Russia’s Interfax news agency said.

Kyrgyzstan’s security service said on Tuesday the likely suspect in Monday’s blast was a Kyrgyz-born Russian citizen. Kyrgyzstan, a predominantly Muslim Central Asian nation, is Russia’s close political ally and hosts a Russian military airbase.