Police are questioning four people in connection with Thursday's smoke-bomb attacks that shut down the Montreal subway system for hours.

Officers say four individuals presented themselves to police Friday.

Photos of the suspects -- three female and one male -- had flooded newspapers and the Internet, as police released images submitted to them by eyewitnesses.

The attacks, which were carried out in different areas of the city's subway system, shut down the metro at the height of rush hour, creating havoc for Montreal commuters.

Unconfirmed reports suggest the attacks could be linked to a group of militant student protesters who have been protesting the government's proposed tuition increases. Reports say the group feels that the main student protesters are not far-reaching enough in their demands.

It is estimated that the subway closure cost the city several million dollars in lost productivity.

The recent student protests, which at times turned violent, have even prompted the U.S. government to issue a warning to travellers heading to the city.

The warning, issued by the U.S. embassy in Ottawa on Apr. 27, warned Americans of the possibility of "unforeseen violence," "vandalism" and "arrests."

The warning also cautioned travellers of disruptions in traffic and public transportation. The warning stated that although most of the demonstrations were peaceful, they could turn violent.

The U.S. warning prompted talks over what will be the total economic cost of the protests.

With the city's festival season about to get underway, there are concerns that the protests may disrupt Montreal's famous Formula One Grand Prix parties and the comedy and jazz festival.

This week, one group made up of students from the Université du Québec à Montréal criticized the protests for being too peaceful and promised to disrupt the Grand Prix parties in June.