Police said Sergio Serrano, 19, of the 7000 block of West Addison Street, told an off-duty security guard he was carrying the gas mask for his own protection while riding the Blue Line train Saturday afternoon.

Police said he told the guard he believed everyone was supposed to wear gas masks.

But when Serrano, dressed in black military fatigues and wearing a bandanna over his mouth, was detained at the Clark/Lake subway station on a weapons charge for allegedly possessing Chinese throwing stars, which have five pointed steel spikes that can penetrate skin, he changed his story, police said. They said Serrano told them he often used the gas mask to cover up the smell of marijuana. "It was a reverse gas mask. When he smoked marijuana, he would recycle the smoke in his gas mask instead of letting it out," said Chicago Transit Authority President Frank Kruesi.

Authorities tested the liquid inside the vial and determined it was skin lotion.

Serrano was charged with unlawful use of a weapon and released on bond, officials said.

The incident occurred the day after federal officials, citing an "unconfirmed, non-specific" threat, warned Friday that terrorists might have been plotting to attack a major U.S. subway system over Memorial Day weekend.

Kruesi said the CTA was informed of the threat before the general public, and the transit agency stepped up security, which he said already was enhanced after Sept. 11.

A Memphis man was arrested May 21 while taking photographs inside the Red Line subway tunnel near the Roosevelt Road station. Authorities determined Christopher Cyko, 18, was a "rail buff," but CTA officials charged him with criminal trespassing.