By Dave Behrens

A half-pound plastic bomb, jammed through a mail slot, failed to explode Friday in the second bombing attempt at the Spanish Tourist office on downtown Biscayne Blvd.

The compact explosive, powerful enough to wreck the office, was harmlessly ripped apart when its detonator exploded sometime before the office opened at 10 a.m.

The small explosion just inside the doorway apparently was unheard.

The front door mail slot had been sealed with plywood after an unsuccessful bomb try March 19, when another half-pound, homemade plastic bomb was dropped into the office. Bomb squad deputies disarmed it before it could explode.

This time, the bomb was packed in a small plastic spray bottle, according to crime lab supervisor Robert Worsham. The evidence of the detonator explosion was discovered by Spanish Consul J. J. Cano and office director Joaquin Pujol.

Cano said he and Pujol saw "strange looking" debris in the doorway when they entered at 10 a.m.

"Then we saw a metal fuse," Cano said, "and I told Pujol, 'That's a bomb.' "

Police said the bombers kicked in the plywood slab which had been nailed over the mail slot last month.

Apparently the Biscayne Blvd. Postman missed the danger signal early Friday morning. With the door slot reopened, mail was delivered through the slot for the first time since the March 19 bomb attempt.

Several letters were found in the doorway beside the bomb debris, Cano said.

This time, police have a slim lead to the identity of the bombers.

A maintenance employee, who entered the Biscayne Terrace Hotel next door at 9 p.m. Thursday, told police he saw two men standing in the tourist office doorway.

"Just as I passed them, I heard a kicking sound coming from the door," he told detectives.

The maintenance employee, who said he was Cuban and asked that his name be withheld, said one of the dark-haired pair was "a 17 or 18-year-old with dark complexion, the other a light-skinned man about 25." Both wore tattered pants, he said.

"I am for Cuba but I have a family," he said later. "That thing could have blown my head off. If they want to fight they should go to Cuba."

Cano refused to speculate on the reason for the two bomb tries.

However, the incidents fit the same pattern of bombings in Greater Miami in recent months involving agencies of governments maintaining political or economic relations with the Castro regime.

Detectives from the crime scene lab worked for more than an hour picking up bomb debris off the slightly scarred rug in the doorway by hand with tweezers.

A travel poster, showing a serene Spanish courtyard, hanging on the wall beside the doorway, was undamaged.