An array of explosives found in a drainage canal on Monday morning forced the closure of a roadway and businesses near the Placentia and Anaheim border as authorities detonated the items and began their investigation into who left them in the area.

The suspicious material – which included hand grenades, grenade fuses, blasting caps and powder – were likely in the area for months, and were probably placed there by someone with knowledge about explosives, authorities said.

A county employee working in the drainage canal under Lakeview Avenue near Orangethorpe Avenue discovered a suspicious backpack shortly before 9 a.m., said Jim Amormino, a spokesman for the Orange County Sheriff’s Department.

Lakeview was closed, businesses in an adjacent industrial park were evacuated and the area was cordoned off for hours as the Sheriff’s bomb squad and FBI bomb technicians dealt with the explosives.

Six grenades were found in the backpack, several that appeared to be live while others were filled with a black powder. Other types of ordnance were underneath the backpack and partially buried in the ground nearby. Authorities also discovered a “fair” amount of blasting powder.

Given the volatile nature of the items, which Amormino described as “horribly unsafe” to transport, authorities elected to destroy them at the scene.

A loud explosion rang through the area shortly after 1 p.m., as the bomb squad destroyed the grenades inside a containment vessel. About an hour and a half later, bystanders and members of the media were told to duck behind vehicles and structures as the blasting caps, powder and other material were destroyed.

Once the explosives were disposed of, the bomb squad and an FBI bomb technician carried out a search of the area. No additional items were discovered.

“They did a very thorough search,” Amormino said. “Obviously we didn’t want any kids finding anything out there.”

With the explosives destroyed, authorities said the next step is determining who left the items in the area and what their motive was, an investigation that will be handled by the Sheriff’s and the FBI.

“It seems to be intentional, just by the sheer number (of devices),” Amormino said.

An ordinary person would “probably not” be able to acquire the explosives, Amormino said. Someone in the military could, and Amormino said investigators are looking into the possibility that whoever left the explosives had some connection to the military. He was quick to add, though, that it could also be a civilian with an interest and knowledge of military-grade explosives.

Authorities have not publicized all of what they found in the drainage canal, which included “different components” that they declined to describe in more detail. Authorities indicated that “other physical evidence” was found in the area, but declined to elaborate.

The “school-type” backpack appeared to be in a deteriorating condition, Amormino said, signaling that it could have been there for months, possibly longer. It was weathered, and had been exposed to moisture.

“But the longer it’s been there, the more volatile it becomes,” Amormino said.

Jose Martinez, owner of JM Tree Service adjacent to where the explosives were found, said he had placed a surveillance camera in his parking lot after a series of break-ins. The camera may also have picked up footage of the drainage canal, and Martinez believed that past footage may be available.

Investigators plan to review the footage. Authorities have not identified any suspects.

Contact the writer: shernandez@ocregister.com or 949-454-7361, or Twitter: salhernandez