Prosecutors said that when McKenzie-Gude learned last week that detectives wanted to search his house, he panicked and drove to White Flint Mall. At his house, police found more than 50 pounds of chemicals, assault-style weapons and armour-piercing bullets.

At a parking garage outside Bloomingdale’s, authorities said, he got out of his sport-utility vehicle and walked up to a 78-year-old man trying to lock his car. McKenzie-Gude demanded the keys, police said. When the man refused, McKenzie-Gude struck the man with his elbows, knocked him to the ground and repeatedly struck him to prevent him from standing, the arrest affidavit stated. McKenzie-Gude took the keys from the man, but could not start the car and fled, police said.

At St John’s, authorities said, McKenzie-Gude got to know the 18-year-old student, who was a year behind him. Sometime last year, the two tested pipe-bombs on three occasions in a Gaithersburg field, police said.

Peter Feeney, assistant state’s attorney, said a pattern can be seen in the cache of guns and other items found in McKenzie-Gude’s house: Two AK-47’s, two bulletproof vests, a single 9-mm weapon but a mention to buy another. “Everything was in two’s,” Feeney said. Police also found “a kind of a to-do list of items to be bought by October of 2008,” Feeney said. The list included “equipment to convert semi-automatic rifles to fully automatic rifles,” Feeney said.