Porter Ranch Blast Called Vandalism : Crime: Three juveniles arrested in the December bombing weren’t protesting the $2-billion project, police say.

Three teen-agers arrested in a recent bombing of construction equipment in Porter Ranch were engaged in vandalism and were not protesting a proposed $2-billion development in the area, Los Angeles police announced Monday.

A second incident, in which an unexploded bomb was found near another construction site in the area, remains under investigation, police said.

The three youths were arrested Thursday on suspicion of exploding an improvised device, police said.

Police said the incident occurred Dec. 20 when a small explosion damaged a front-loader parked on a construction site on Sesnon Boulevard near Louise Avenue.

A homemade device exploded on the seat of the vehicle owned by M.J. Brock & Sons Construction, damaging the seat and shattering the windows.

Police declined to say what evidence led them to the youths, who live in the area and were released to their parents.

Detectives said they found no evidence linking the incident to the proposed 1,300-acre Porter Ranch development being considered by the Los Angeles City Council.

“The motive for the bombing was vandalism,” said Detective Ken Bourne of the Criminal Conspiracy Unit. “There were no political overtones to the incident.”

Bourne said detectives are continuing to investigate a separate bomb incident Jan. 5, in which an unexploded device was found under a flatbed truck on Sesnon Boulevard near Tampa Avenue.

The device was discovered by a construction site security guard when it burst into flames but failed to explode, police said.

Bourne said there was no evidence linking the two bombs or connecting the second incident to the Porter Ranch proposal, which, if approved by the council, would become the largest single development in the city’s history.