Police closed a road in front of a Mill Woods elementary school Monday afternoon as they investigate a suspicious package, just days after explosive devices were found in the field of the same school.

Police say a 33-year-old man was arrested on Monday.

The man was carrying a backpack and police say he told officers there were explosives in it.

Officers believe the man is involved in a similar incident involving explosives over the weekend.

Police began using a robotic device to inspect the backpack around 3 p.m. Monday.

The backpack was near a bus stop by Ecole Frère Antoine Elementary School at at 2849 Mill Woods Rd.

Police say the type of explosive they believe is involved is most often used for sound effects rather than hurting people.

Charges are now pending and police say the suspect is co-operating.

On Saturday, a bomb squad was called to the school after the discovery of "homemade explosive devices."

When they arrived, police say they found a number of "homemade explosive devices."

On Monday, the school sent a letter to parents advising them that "potentially toxic and flammable chemicals" were left in the school field over the weekend.

"Police investigated and discovered that the bottles were filled with some chemicals that could potentially cause harm to someone if they were mixed together. The Edmonton Police Service continues to investigate and search for the individual(s) responsible," the letter by Principal Danielle Fortier stated.

"This morning we held a school assembly to let children know about the suspicious items and ask that they not pick up anything in the playground but to let a trusted adult know about the item."

Parents tell CTV News the school was under lockdown for about 30 minutes Monday.

Students were out for recess when police started closing the area down to cars and foot traffic.

Some parents described the situation as "terrifying."

Others questioned why they weren't informed about the incident over the weekend.

"I don't know why we weren't told there were explosives in the playground before we sent them to school," one parent told CTV News.

"It bothers me a bit because there could be more obviously so I would have liked to know that before they went to school today."

Lori Nagy with Edmonton Catholic Schools says the school notified students and parents as soon as it was aware of the incident.

"The school only found out about the incident today," Nagy said.

"That's when the school decided to hold an assembly with the children, let them know what's happened in the past days and also let children know that they are safe."

Police asked commuters travelling in the area of Lakewood Road and 28 Avenue to take a different route for most of the day.

The area was re-opened around 7 p.m. Monday.

With files from Sean Amato