A 25-year-old Arlington County man was arrested after threatening on his Facebook page to use explosives in the Washington area, writing that he could put pipe bombs on Metro cars or in Georgetown at rush hour, according to a criminal complaint filed in federal court in Virginia.

Court papers do not indicate that Awais Younis, also known as Mohhanme Khan and Sundullah Ghilzai, ever acted on the threats. He has been charged with communicating threats via interstate communications.

In conversations with another Facebook user, Younis described how to build a pipe bomb and indicated what type of shrapnel would cause the most damage. He talked about putting bombs on the third and fifth cars of a Metro train, which he said held the largest number of passengers. In one posting, he said, "Christmas trees were going to go boom."

Younis's case is the second in recent days in which Facebook has pointed authorities toward suspects in terrorism investigations. Federal authorities cited the popular social networking site in the case against a Baltimore man accused of plotting to blow up a military recruiting center. Authorities said they learned of Antonio Martinez's radical leanings on Facebook, joined his plot and supplied him with a fake car bomb that he tried to detonate last week.

Arthur Hulnick, a Boston University professor who worked with the CIA for 28 years, said the most serious and deadly terrorists want to act in secret. But he said authorities must devote resources to checking out threats to identify those that might be serious.

"A real terrorist who is going to blow up the Washington Metro wouldn't put an advertisement on Facebook. He'd just do it," Hulnick said. "But they have to check it out. You can't ignore it."

Younis came to the attention of authorities in late November when a Facebook user corresponding with a man known as Sundullah Ghilzai became worried and contacted FBI agents in New Orleans. FBI Special Agent Joseph J. Lesinki described the online contact between the two in an affidavit filed in U.S. District Court in Alexandria.

Ghilzai wrote that a bomb in Georgetown at rush hour would cause the greatest number of casualties, according to the affidavit. He wrote that he could put pipe bombs on Metro train cars without being noticed. When the correspondent replied, "you wouldn't do that," Ghilzai answered, "watch me," the affidavit said.

Authorities obtained a search warrant for the Facebook account that Ghilzai used and determined that his legal name is Awais Younis, court papers said.

On Dec. 5, the person who had conversed with Ghilzai again reached out to the FBI. The two had chatted again through Facebook, and Ghilzai seemed angry and agitated.

"You are sticking your nose where it doesn't belong into something bigger then you and I," he wrote. "that is the problem with Americans they cant leave well enough alone until something happends then they sit there wondering why we dropped the twin towers like a bad habit hahaha."

The correspondent said Ghilzai posted a photo on his Facebook page showing himself holding an AK-47 rifle and standing with his uncle in front of a tent filled with explosives. The caption read: "My family business," according to court papers. Another photo, with the caption "bullet behind every rock," showed a hand holding rifle rounds.

Ghilzai also mentioned the correspondent's father, who lives in the Washington area and takes Metro, in a menacing way. "Do yourself a favor and tell your father to cancel work tomorrow," he wrote.

Younis was arrested Dec. 6, and the case was unsealed Dec. 9. A judge has ordered a mental health evaluation before a court hearing scheduled for this month, authorities said.

The arrest came months after authorities charged Farooque Ahmed, 34, of Ashburn with conspiring with people he thought to be al-Qaeda operatives to bomb the Arlington Cemetery, Pentagon City, Crystal City and Court House Metro stations. The operatives were actually working with the FBI.